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Full text of "Bulletin of the British Museum (Natural History), Mineralogy"

THE PSEUDOLEUCITE 
BOROLANITES AND ASSOCIATED 
ROCKS OF THE SOUTH-EASTERN 

TRACT OF THE 
BORRALAN COMPLEX, SCOTLAND 



A. R. WOOLLEY 



BULLETIN OF 
THE BRITISH MUSEUM (NATURAL HISTORY) 
MINERALOGY Vol. 2 No. 6 

LONDON: 1973 



THE PSEUDOLEUCITE BOROLANITES 

AND ASSOCIATED ROCKS OF THE 

SOUTH-EASTERN TRACT OF THE 

BORRALAN COMPLEX, SCOTLAND 



BY 

ALAN ROBERT WOOLLEY 



Pp. 285-333 J 6 Plates, 15 Text-figures, 9 Tables 



BULLETIN OF 
THE BRITISH MUSEUM (NATURAL HISTORY) 
MINERALOGY Vol. 2 No. 6 

LONDON: 1973 



THE BULLETIN OF THE BRITISH MUSEUM 

(natural history), instituted in 1949, is 
issued in five series corresponding to the Departments 
of the Museum, and an Historical series. 

Parts will appear at irregular intervals as they 
become ready. Volumes will contain about three or 
four hundred pages, and will not necessarily be 
completed within one calendar year. 

In 1965 a separate supplementary series of longer 
papers was instituted, numbered serially for each 
Department. 

This paper is Vol. 2, No. 6 of the Miner alogical 
series. The abbreviated titles of periodicals cited 
follow those of the World List of Scientific Periodicals. 



World List abbreviation : 
Bull. Br. Mus. nat. Hist. (Miner.) 



Trustees of the British Museum (Natural History), 1973 



TRUSTEES OF 
THE BRITISH MUSEUM (NATURAL HISTORY) 

Issued 16 August, 1973 Price £2.95 



THE PSEUDOLEUCITE BOROLANITES 

AND ASSOCIATED ROCKS OF THE 

SOUTH-EASTERN TRACT OF THE 

BORRALAN COMPLEX, SCOTLAND 

By A. R. WOOLLEY 

CONTENTS 



I. 


Introduction ...... 






Page 
288 


II. 


Previous work ...... 






288 


III. 


Field relationships ..... 






289 


IV. 


Petrography ...... 

(i) Pseudoleucite suite .... 

(a) Muscovite group .... 

(b) Biotite-magnetite group 

(c) Borolanite ..... 

(d) Xenoliths ..... 

(e) Summary ..... 

(2) Lower suite : the Aultivullin Quarry boreholes 

(a) Pyroxene group .... 

(b) Granular borolanite .... 

(c) Hornblende group .... 

(3) Lower suite : the Allt a'Mhuilinn section 

(a) Vullinite ..... 

(b) Shonkinite ..... 

(c) Borolanite ..... 

(d) Andradite-biotite syenite . 

(4) Other rock types ..... 

(a) Leucocratic borolanite 

(b) Aegirine-^nepheline-analcime borolanite 

(c) Dykes and veins .... 






294 
294 
294 

295 
296 
298 
299 
300 
301 
302 
303 
304 
304 
305 
305 
306 
306 
306 
306 
307 


V. 


Alkali feldspars ...... 






308 


VI. 


Chemistry ....... 






312 


VII. 


Discussion ....... 

(1) The pseudoleucite suite .... 

(2) The lower suite ..... 

(3) The relationship to the ledmorites . 






325 
325 
327 
329 


VIII. 


Acknowledgements ..... 






33i 


IX. 


References ...... 






33i 



SYNOPSIS 

The contact of the rocks at the southeastern end of the Complex with the later syenites is 
an intrusive one. The South-eastern Tract comprises an upper pseudoleucite suite and an 
earlier, underlying lower suite. Within the pseudoleucite suite are three units : at the top 
are potassic feldspar-muscovite rocks, beneath these are potassic feldspar-biotite-magnetite 
rocks, beneath which are borolanites (potassic feldspar-melanite-biotite + nepheline). 



288 PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 

Pseudoleucites are present throughout, but no intrusive junction has been recognized within 
the sequence. The potassic feldspars change from orthoclase in the borolanites through to 
microcline in the muscovite group. The lower suite has been encountered in boreholes down 
to 151 feet (46-02 m) in the Quarry, and the upper part outcrops in the Gorge at Aultivullin. 
It consists of an upper series of pyroxene-biotite and andradite-biotite syenites (the pyroxene 
group), which includes the rock vullinite ; a middle group of pseudoleucite-free borolanites, 
and a lower group of hornblende, hornblende-pyroxene and andradite-biotite syenites (the 
hornblende group). Xenoliths in the pseudoleucite borolanites derive from the pyroxene 
group. 

Twenty-one new whole rock analyses, nineteen analyses of rocks for alkalis only, together 
with an andradite, three pseudoleucites, two nephelines and two feldspar analyses are pre- 
sented. There is an upward increase in the pseudoleucite suite in silica, alumina and potash, 
while iron, lime, titania and soda decrease upwards. The muscovite group shows exceptional 
enrichment in potash with values generally greater than 14 per cent and soda less than 0-5 per 
cent. The observed chemistry and mineralogy are referred to crystallization from the bottom 
upwards of soda-leucite and gravitational settling of melanite and pyroxene. The pyroxene 
and hornblende groups of the lower suite are richer in soda than the pseudoleucite suite and 
comparable with the ledmorites. They are intruded and metasomatized by the borolanites 
and by the later syenites, the former causing potash enrichment and the growth of melanite, 
the latter an increase in silica, the ' hornblendization ' of pyroxene and the replacement of 
pyroxene and hornblende by andradite and biotite. The pseudoleucite suite, the lower suite, 
and the ledmorites are probably derived from a common parent, the pseudoleucite suite rep- 
resenting upward concentration in soda-leucite crystals, and the lower suite and ledmorites 
produced by enrichment by gravitational settling of pyroxene. This gravitational differen- 
tiation took place prior to emplacement. 

I. INTRODUCTION 

Recent drilling by the Robertson Research Company at the eastern end of the 
Borralan Intrusion encountered a series of rocks the majority of which are not 
exposed at the surface. A re-examination of the rocks from the exposed part of 
the Complex at its eastern end, both in the field and in the laboratory, has revealed 
that there is much greater diversity among the described rocks than has been 
realized hitherto, and has led to the recognition of rock types not described before 
from the Complex. The purpose of the present paper is, therefore, to present an 
account of this suite of rocks, and to give detailed descriptions of the new rock 
types discovered by drilling. The exposed rocks are re-described with particular 
reference to their variations and inter-relationships. The only published chemical 
data on the rocks at the eastern end of the Borralan Intrusion are five borolanite 
analyses. Some 29 new rock and mineral analyses and a number of partial analyses 
are presented here and these, together with the field and petrographic data, have 
led to a re-appraisal of the petrogenesis of these rocks. 

II. PREVIOUS WORK 

It was in the middle years of the nineteenth century that mention was first made 
in the literature of the rocks of the Loch Borralan area, although this was largely 
incidental to the interest and controversies that were developing over the structure 
of the North-west Highlands. The first specific account was by Home and Teall 
(1892) who described, figured and named the rock borolanite. In a later paper 



FROM THE BORRALAN COMPLEX, SCOTLAND 289 

Teall (1900) described a number of other Borralan rocks. The first map was 
produced as a result of the systematic sheet mapping by the officers of the Geological 
Survey (1 inch to 1 mile sheets 101, 1892 ; 102, 1925 ; special Assynt Sheet, 
1923) ; the related account is contained in the Memoir (Peach et al. 1907). It 
is, however, in the various papers of Shand (1906, 1909, 1910, 1939) that the most 
detailed descriptions of the Complex and its constituent rocks are given. A short 
paper on borolanite was published by Smith (1909), while certain aspects of pedo- 
genesis were dealt with by Bowen (1928), based on Shand's descriptions, and 
Phemister (1930) described a carbonated ultrabasic rock. A general account of 
the area is given in the geological excursion guide to the Assynt District and 
Sutherland (Macgregor & Phemister 1937). Further contributions were made by 
Sabine (1953), Tilley (1958) and Woolley (1970). Unpublished work is contained 
in theses by Phemister (1929) and Woolley (1965). 

Several of the minerals of the Borralan Complex have aroused interest including 
sulphatic cancrinite and analcime (Stewart 1941), soda-pyroxene (Sabine 1950), 
nepheline (Tilley 1956) and opaque minerals (Stumpfl 1961). 

Shand (1910) suggested that the nature and concentric distribution of the Bor- 
ralan rocks could be accounted for by mechanisms of limestone assimilation and 
gravitational differentiation in place, resulting in lower more basic layers grading 
upwards into syenites and quartz syenites. The rocks at the eastern end of the 
complex, designated the South-eastern Tract by Shand (1910, p. 381), were thought 
to have been thrust such that these relatively basic rocks were juxtaposed with 
syenites and quartz syenites of a higher level (op. cit. p. 381). He recognized three 
' belts ' of rocks in the South-eastern Tract : a belt of schistose rocks (the vullinite 
exposures), a belt of ' spotted and unspotted borolanites ' running from the Gorge 
to the Knolls (Text-fig. 2), and a belt of ' granulites ' overlying the borolanites. 
Although Shand recognized spotted rocks free of garnet among the granulites, he 
considered that the granulites were the product of granulation and hydrothermal 
alteration of borolanite (Shand 1910, p. 412 ; 1939, p. 415). 

The rocks of the Borralan Complex can be divided into an earlier and later series 
separated by an intrusive junction (Text-fig. 1). The later rocks are syenites and 
perthosites grading into quartz syenites. The earlier series comprises pseudo- 
leucite types, including the well-known borolanite, ledmorites, nepheline syenites 
and various ultramafic rocks. 



III. FIELD RELATIONSHIPS 

The South-eastern Tract is limited to the east by eastward dipping marbles, to 
the south by thrust quartzites of the Assynt Nappe (Sabine 1953, p. 152), and to 
the west by an intrusion of syenite (Text-fig. 2). Mapping suggests that the 
eastern contact, although nowhere exposed, is probably conformable with the 
marbles and dips gently eastwards. A small outcrop of marble 500 m west of 
the limestone boundary (Text-fig. 2) may represent a xenolith, or part of a roof 
pendant. The southern contact is also unexposed but indirect evidence, in the 
form of a series of alkaline dykes cutting the thrust quartzite and exposed in the 



2 go 



PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 




Ledmorites 



Ultramafic rocks 



Nepheline syenites 



Later syenites 



Fig. i. The general geology of the Borralan Complex. The location of Text-fig. 2 is 

indicated. 



streams flowing northwards off Glas Choille, together with the presence of meta- 
somatized quartzites, also at Glas Choille, suggests the emplacement of the igneous 
rocks after the main period of thrusting. The intrusive contact with syenite is 
exposed at the southern end of the syenite apophysis south-south-east of Loch 
a'Mheallain (Text-fig. 2). The porphyritic syenite is somewhat finer grained close 



FROM THE BORRALAN COMPLEX, SCOTLAND 



291 



Later Syenites 

Marbles ond limestones 

Assynt and Moine nappes 

Borolanite 

Biotite - magnetite group 

Muscovite group 



L B leucocratic borolanite 

A B Andradite borolanite 

Sh Shonkinite 

FQ Fenitised quartzite 

Vol Vullinite 



Thrust 




Fig. 2. The geology of the South-eastern Tract. Line of section (Text-fig. 3) is indicated. 



292 



PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 



to the contact and cuts sharply across rocks containing distorted pseudoleucites. 
This contact was mapped as a thrust by Shand (1910, p. 381) and a fault is shown 
along part of the contact on the published one-inch map. Veins of syenitic com- 
position are abundant in the Quarry (see Text-fig. 2), the Gorge and cutting the 
rock designated as vullinite on Text-fig. 2. 

Recognizable pseudoleucites are abundant over the whole of the South-eastern 
Tract, though they are commonly distorted. Among the rocks of the Knolls and 
in a north-south trending belt in the central part of the Tract, dark, eastward- 
dipping, platy rocks appear to represent an extreme stage of deformation in which 
the pseudoleucites have been obliterated. It is, however, common to find quite 
undistorted pseudoleucites next to strongly sheared rocks. 

It is possible to map a line across the South-eastern Tract separating garnet- 
bearing rocks (borolanites) to the west from garnet-free rocks to the east (Text- 
fig. 2) . A few patches of each type lie on the ' wrong ' side of the line, but gen- 
erally this boundary could be mapped with confidence because of the easy recog- 
nition of garnet in hand specimen. A consideration of the trend of this boundary 
in relation to the topography suggests that it is fairly planar and dips eastwards 
at an angle which is just greater than the slope of the ground (Text-fig. 3). No 
evidence has been found to suggest that this boundary is an intrusive one, nor 
are the rocks on one side of it more deformed than on the other, and pseudoleucites 
are equally abundant on either side. Another boundary (Text-fig. 2) separating 
a muscovite group from a biotite-magnetite group of pseudoleucite rocks is based 
mainly on a study of thin sections, although these rocks also have distinctive 
features in the field. 



Gorge 




PSEUDOLEUCITE SUITE 
Muscovite group 
["XvXl Biolire - magnetite group 



(■— l *tW Pyroxene group [includes 

r- -H vullinite ond shonkinite] 



Gronular borolanite 



Later syenite 
Limestone 



U_ ^ I Hornblende group 



Fig. 3. Section across the South-eastern Tract. Line of section indicated on Text-fig. 
2. The position of the Aultivullin Quarry boreholes are indicated. 



Rocks beneath the pseudoleucite suite are exposed in the steep walls of the Gorge 
at Aultivullin. In the eastern bank of the northern (upper) end of the Gorge the 
lower limit of the pseudoleucite rocks can be distinguished ; it is fairly sharp, 
although there is considerable shearing which has obscured the real nature of the 
boundary. Below this contact are eastward dipping foliated rocks consisting of 
a deeply weathered shonkinite and a pseudoleucite-free borolanite. The borolanite 
and shonkinite form interleaving layers and pods of variable thickness, the shon- 
kinite becoming thinner to the north. To the south both rock types are lost 



FROM THE BORRALAN COMPLEX, SCOTLAND 293 

beneath scree. The southern (lower) half of the Gorge exposes massive andradite- 
bearing borolanites which probably he beneath the shonkinite-borolanite group. 
The few outcrops immediately west of the Gorge are of pseudoleucite borolanites, 
and these are of a distinctive pink colour in contrast to the usual grey. The pseudo- 
leucite-free rocks along the Gorge were considered by Shand (1910, p. 409) to result 
from shearing of pseudoleucite borolanites. 

The rock termed ' vullinite ' by Shand (1910, p. 407) crops out along the Allt 
a'Mhuilinn to the north of the Gorge. It forms eastward dipping slabs which are 
veined and eventually cut out by the syenite intrusion. The spatial relationship 
of the vullinite to the pseudoleucite rocks is nowhere apparent, but it is noteworthy 
that the vullinite is on the same strike as the similarly dipping foliated rocks be- 
neath the pseudoleucite borolanite of the Gorge. 

Three boreholes put down in the Quarry by the Robertson Research Company 
have brought to light a completely new group of rocks, and helped in understanding 
the equivocal relationships at the Gorge, and the probable role of vullinite in the 
geological setting of the South-eastern Tract. The holes were drilled within a few 
metres of each other and reached 159, 151 and 90 feet (48-46, 46-02 and 27-43 m) 
respectively. A detailed petrographic account of these rocks is given later, but 
the principal rock groups encountered can be summarized as follows : 

0-20 feet (o-6-io m) Pseudoleucite borolanite 

20-50 feet (6-10-15-24 m) Mixed pyroxene-bearing group, includes andra- 

dite-pyroxene syenites, and vullinite 
50-90 feet (15-24-27-43 m) Pseudoleucite-free borolanites 
90-156 feet (27-43-47-55 m) Mixed hornblende-bearing group 

A very noticeable reddening is apparent in the two longer cores, particularly in 
the bottom 10-15 Iee * (3'°5~4'57 m ) where it is associated with red syenite veins, 
but it first becomes noticeable at depths of about 120 feet (36-58 m). This effect 
is very similar to contact phenomena at the margin of the later syenite in the 
Ledmore area of the Complex (Woolley 1970, p. 177), and suggests that a contact 
with syenite occurs close to the bottom of the two deeper holes, as has been assumed 
in Text-fig. 3. The same process undoubtedly caused the reddening of borolanites 
west of the Gorge. 

The contact between the pseudoleucite borolanite and the underlying rocks in 
the boreholes is comparatively sharp, but, as in the Gorge, is obscured by shearing. 
The boreholes show that the rocks lying below the pseudoleucite borolanite in the 
Gorge, together with the vullinite, extend eastwards beneath the main outcrop of 
pseudoleucite rocks (Text-fig. 3). How far eastwards they extend cannot be deter- 
mined without further drilling. 

The cliffs to the north-north-east of the Quarry are of a leucocratic borolanite 
free of pseudoleucite (Text-fig. 2) and at the northern end of the cliffs there is a 
sharp contact with pseudoleucite borolanite. The leucocratic borolanite appears 
to vein the other, while the vague outlines of a large raft-like mass of the pseudo- 
leucite borolanite can be traced within the leucocratic type. 



294 PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 

In the vicinity of Glas Choille (Text-fig. 2) a small mass of metasomatized 
quartzite (Woolley et al. 1972) probably represents a quartzite from the Assynt 
Nappe, but lack of exposure obscures the relationship of this rock to pseudoleucite 
rocks of the South-eastern Tract. 

IV. PETROGRAPHY 

For descriptive purposes the South-eastern Tract rocks are divided into a 
' pseudoleucite suite ', and a ' lower suite ' which comprises the rocks lying struc- 
turally below the pseudoleucite borolanites in the Gorge and the boreholes. A 
number of rocks cannot be accommodated in either category and are treated 
separately. 

(1) Pseudoleucite suite 

The pseudoleucite suite is subdivided into three groups in which the feldspar is 
exclusively, or overwhelmingly, potassic. The mafic minerals define the following 
assemblages : 

muscovite (magnetite) (Muscovite group) 

biotite - muscovite - magnetite (Biotite-magnetite group) 
melanite - biotite (Borolanite) 

These assemblages occur in this order from east to west across the Tract (Text- 
fig. 2), and they are described accordingly. 

(a) Muscovite group 

The rocks of the muscovite group are usually pale grey or pink. Rare patches 
and streaks of mafic minerals, together with muscovite, sometimes define vague 
pseudoleucites which may be enhanced on weathered surfaces to form rounded, 
wart-like knobs up to 2 cm in diameter. Potassic feldspar and muscovite, some- 
times with the addition of carbonate, usually constitute about 99 per cent modally. 
One specimen is outstanding in containing some 80 per cent of muscovite [1972, 
P8 (249)] 1 . The potassic feldspars are rather variable in size but usually build 
either large anhedral plates, up to 2 cm in diameter, or a much finer-grained mosaic 
which is patchily developed and sometimes included within the large plates. Small 
grains commonly show microcline cross-hatching but large ones rarely so. Musco- 
vite forms coarse and fine-grained aggregates up to a centimetre across, and ragged 
isolated flakes which may be interstitial to, or enclosed by, feldspar (PI. 16, 3) ; it 
also fills narrow veinlets. Biotite, pleochroic from yellow or greenish-yellow to 
apple-green or deep khaki-green, occurs as ragged individual flakes and clusters up 
to 7 or 8 mm across, which are usually included and replaced by feldspar. An 
association of biotite and ore is often evident and biotite sometimes appears to be 
altering to muscovite. Magnetite euhedra, up to 1 mm across, partially or wholly 
altered to hematite (martite), are common, while veins and aggregates of carbonate 
are abundant in the Alltan nam Breac vicinity (Text-fig. 2). Oligoclase has been 
identified in one section [1972, P8 (131)] and apatite and sphene, enclosed in feld- 
spar, are also found. 

1 Numbers in brackets are British Museum (Natural History) rock collection specimen numbers. 



FROM THE BORRALAN COMPLEX, SCOTLAND 295 

The leucite pseudomorphs are difficult to distinguish in thin section as they have 
textures identical with the matrix. They are also similar mineralogically being 
composed essentially of aggregates of potassic feldspar and muscovite, while 
biotite, ore and apatite are confined to the matrix. 

(b) Biotite-magnetite group 

The pseudoleucites in these rocks vary considerably in size from a maximum 
of about 2 cm in diameter to 1 mm or less ; they may be angular, rounded, lenticu- 
lar or vague ; approximately equal in size, or variable within one hand specimen ; 
and they may be rare or abundant. The colour is also variable from pinks to greys 
or white. The lenticular pseudomorphs have been formed by shearing and flat- 
tening, and they become vague as these rocks grade eastwards into the muscovite 
group. A more rare type of pseudomorph, suggestive of prismatic sections of 
amphibole or pyroxene, is black, up to 4 mm in diameter and sometimes rectangular 
(PI. 16, 1 & 2)." 

Rocks of the biotite-magnetite group contrast with the muscovite group in being 
finer grained, for the feldspars, biotite and ore diameters of between o-oi and 
o-i mm are usual, though towards the muscovite-rich rocks a slight coarsening is 
sometimes evident. Both the pseudoleucites and groundmass are finely granular, 
though the pseudoleucites are usually slightly coarser. 

Pseudoleucites can be distinguished in thin section by their freedom from biotite 
and ore, and sometimes by their slightly coarser texture. The edges of the pseudo- 
morphs are commonly very sharp and the occasional concentration of ore along the 
boundary enhances the contrast. They are composed of potassic feldspar, musco- 
vite and a little carbonate. The muscovite content varies from 10 to 100 per cent, 
and tends to be inversely proportional to the size of the pseudomorph. The fine- 
grained but undistorted nature of many of the pseudomorphs indicates that the 
fine grain size is not a product of shearing. 

The potassic feldspar of the matrix forms an equigranular mosaic which is some- 
times modified in the more easterly rocks, by the development of larger individuals. 
In some sheared rocks the feldspars develop a dimensionally orientated texture in 
which they have sometimes been noted to ' flow ' around the pseudomorphs as if 
these had acted as resistant augen. Occasionally grains of oligoclase are present 
and these are especially abundant in the vicinity of the tongue of syenite which 
encroaches into the South-eastern Tract to the northeast of Aultivullin. 

Biotite has two quite distinct habits. Westwards towards the borolanite biotite 
is abundantly present as innumerable tiny, fresh, euhedral, green flakes, which 
often appear to be developing at the expense of muscovite. Eastwards, however, 
towards the outer contact of the Complex, biotite decreases in amount and is 
present as large, often ragged, grains and clusters which show alteration to muscovite 
and replacement by feldspar. Some of these clusters of coarse biotite correspond 
to the black pseudomorphs sometimes apparent in hand specimens. The magnetite 
is also of two types ; to the west the ore forms tiny, euhedral to subhedral grains 
which are evenly distributed, but on the eastern side of the Tract it occurs domi- 
nantly, as irregular masses and aggregates in rocks in which it is often modally 



2Q6 PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 

more abundant than biotite. Carbonate may comprise as much as 7 per cent of 
the rock, in the form of veins, aggregates, large isolated plates and as a fine pep- 
pering in the feldspars. Deep blue to colourless fluorite is found in both pseudo- 
morphs and matrix of some rocks near to the borolanites. Sphene also is restricted 
to the area near to the borolanite junction, as tiny pale-yellow grains, often within 
biotite. Apatite, epidote and pyrite also occur. 

Some members of this group lack pseudoleucites, and these rocks closely resemble 
the matrix of the pseudoleucite-bearing rocks. The dearth of pseudomorphs seems 
often to be due to their obliteration by recrystallization, which is sometimes 
demonstrably associated with shearing, but usually there is little trace of textures 
which are obviously diagnostic of deformation. There is no doubt that some of 
these rocks were deformed before and some after recrystallization and development 
of the newer biotite and magnetite. 

(c) Borolanite 

The pseudoleucite borolanites are grey rocks speckled with white, and sometimes 
pink, pseudoleucites up to 2-5 cm in diameter. Black, often euhedral, garnets as 
much as 0-5 cm across are studded through the matrix but do not occur within the 
pseudomorphs. The pseudoleucites are megascopically identical to those of the 
biotite-magnetite group rocks and show all stages of deformation to thin white 
streaks though in some instances the associated garnets are apparently unaffected 
by the movements. However, other borolanites with deformed spots contain 
strongly sheared garnets. Unlike the biotite-magnetite group, sharply defined 
angular pseudomorphs are rather rare among the borolanites. The borolanites 
contain melanite, nepheline, cancrinite and zeolite, all of which are absent from the 
biotite-magnetite and muscovite group rocks, while magnetite is relatively rare 
and muscovite reduced in importance. Garnet, magnetite and biotite rarely occur 
within the pseudoleucites but the other minerals occur indiscriminately in pseudo- 
morphs and matrix. 

The feldspar of the borolanites is an orthoclase, though some degree to triclinicity 
is often manifest (see section V). It forms large anhedral plates which poiki- 
litically enclose the other minerals (PI. 17, 1 & 2), but towards the contact with the 
biotite-magnetite group it builds a fine-grained mosaic (PL 16, 4). The fine, 
granular feldspar is identical to that of the biotite-magnetite group and in some 
specimens the finer-grained feldspar is restricted to patches which are interstitial 
to, and sometimes enclosed by, the more usual poikilitic plates [1972, P8 (121), 
(123) & (285)], and the large crystals appear to have overgrown and replaced the 
finer grained feldspar. These rocks represent the transition from textural types 
typical of the biotite-magnetite group to those typical of the borolanites. The 
larger feldspar plates as well as poikilitically including biotite and garnet, overgrow 
the pseudoleucites (PI. 17, 1 & 2). Plagioclase is uncommon in the borolanites 
but oligoclase is found in the western part of the South-eastern Tract, close to the 
syenite contact. 

Melanite, the second ubiquitous mineral of the borolanites, forms between 20 
and 30 per cent of the matrix. It shows all degrees of idiomorphism and varies in 



FROM THE BORRALAN COMPLEX, SCOTLAND 



297 



size up to about 0-5 cm. The colour ranges from honey-yellow to dark brown, and 
simple zoning, from dark core to pale rim, or multiple zoning may be developed. 
Analyses indicate about 4 per cent of Ti0 2 for these garnets (Table I). The garnet 
occurs either as large euhedral to subhedral grains, which are frequently patchily 
altered to sphene, and include or cut across biotite, or as myriads of small yellow 
grains which may be embedded in feldspar but more usually form wormy growths 
in biotite (PI. 17, 3) and sometimes in pyroxene and amphibole. The alteration of 
melanite to sphene may result in an even distribution of hundreds of small sphenes 
through the garnet, or in the patchy development of only a few sphene individuals. 



Table I 

Chemical analyses of melanites and andradite 



Si0 2 
Ti0 2 
ALO, 



33-58 
3-96 
2-61 



3394 
3-88 

3-89 



37-°4 
i-57 
8-13 



A B 1 

(metal ions on the basis of 24 oxygens) 
Si 5-717 5-64 6-076 

Ti 0-283 - - 

Al - 0-36 



Fe 2 3 


24-63 


23-02 


15-88 


Ti 


°-244 


0-48 


0194 


FeO 


1-67 


123 


2-63 


Al 


0-524 


0-40 


1-572 


MnO 


0-69 

0-84 

30-04 


o-53 

0-24 

32-99 


200 
32-50 


Fe 3 + 


3156 


2-88 


1-960 


MgO 


S Y 


3-904 


3-76 


3-726 


CaO 


Fe 2 + 


0-238 


0-18 


0-360 


Na 2 


- 


049 


- 


Mn 


0-099 


0-08 


- 


K 2 


- 


0-21 


- 


Mg 


0213 


0-06 


0-488 


H 2 0+ 


- 


O-II 


- 


Ca 


5-48i 


5-88 


5-312 


H 2 0- 


- 


OOO 


- 


Na 


- 


016 


- 


p 2 o 5 


- 


tr. 


- 


K 


- 


004 


- 



Total 
A 



98-02 



100-53 



99-75 



T.X 



6-031 



6-40 



6-160 



Melanite, from nepheline syenite pegmatite, Aultivullin Quarry (Howie & Woolley 1968, 
Table 1, no. 4) 
B. Melanite from pseudoleucite borolanite, Borralan Complex (Miyashiro 1959, Table 1) 
1. Andradite from an andradite syenite in borehole iA at depth of 36 feet (10-97 m ). Aulti- 
vullin Quarry [1969, P17 (5)]. Analyst : C. J. Elliott 

Biotite forms euhedral to anhedral flakes up to 3 mm in length ; it is pleochroic 
with a medium to pale yellow to pale yellow-green, and /S = y apple-green to deep 
khaki-green. Rarely reddish-brown streaks and patches occur in the biotite and 
brown halos are always found around included apatites; 2V( — ) is very small. 
Biotite together with intergrown garnet sometimes defines rectangular areas [1972, 
P8 (344) & (356)] which are probably pseudomorphs after pyroxene (PI. 17, 4). 
Muscovite forms occasional clusters in the matrix or the pseudomorphs, and a little 
sericite is sometimes evident. 

Nepheline may be present as discrete grains or as a complicated intergrowth 
with feldspar, referred to as dactylotypic texture by Shand (1906, p. 429 & Plate 



298 PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 

XVI). The first type of nepheline is invariably ragged in form, usually included 
by feldspar, and occasionally displays a textural relationship with feldspar which 
suggests replacement, though the direction of this replacement is difficult to 
interpret (PL 18, 3). The dactylotypic texture consists of very fine cylindroidal 
threads which lie within feldspar and have an average diameter of about o-oi mm 
and lengths up to 0-2 mm. They form either radiating or subparallel bundles 
(PL 18, 1). The feldspar may be completely or only partly permeated by this 
structure, and there is some dependence of the development of dactylotypic 
nepheline on the boundaries between the feldspar grains (PL 18, 2). The dactylo- 
typic nepheline is usually sericitized and occurs indiscriminately in pseudoleucites 
and matrix, but there is some evidence from modes that there is a greater proportion 
of nepheline, of both textural types, within the pseudomorphs. Cancrinite tends 
to be associated with nepheline, often as an alteration product, but also occurs as a 
late primary mineral in nepheline-free rocks. Nepheline and cancrinite are restricted 
to the borolanites of the Aultivullin area and immediately to the east of Aultivullin. 

Replacement of feldspar by zeolite, tentatively identified as mesolite and thom- 
sonite, is widespread in the area of the Gorge and Quarry. Pyroxene is rare in 
the pseudoleucite borolanites. It forms discrete, subhedral grass-green grains 
seldom more than 0-05 mm in length ; it is an aegirine-augite. Hornblende has 
been found in the rocks of the Gorge and is common in the small exposures west of 
it [1972, P8 (106-108)] where it develops sturdy, sieved prisms up to 1-5 mm long 
which have the pleochroic scheme a pale-khaki, jS green and y deep blue-green ; 
y : c 31 . Apatite is abundant, but ore is very minor except in the finer-grained 
borolanite near to the biotite-magnetite group. Carbonate, fluorite, epidote and 
andradite garnet have also been identified in the pseudoleucite borolanites. 

The pseudoleucites of many of the borolanites display a greater or lesser degree 
of ellipticity, but they seldom show in thin section any sign of the deformation 
to which they have been subjected. From this it must be concluded that recrystal- 
lization of the constituent minerals took place after, or simultaneously with, the 
deformation as suggested by Home and Teall (1892, p. 176). That the shearing 
movements outlasted the main mineralogical reconstitution, however, is shown 
by the brittle deformation of some rocks of the South-eastern Tract. Moderate 
deformation leads to bent micas, broken garnets and fractured feldspars, but in 
more extreme cases the feldspar is completely comminuted to a fine ' paste ' and 
garnet is streaked out and broken down to ore and carbonate. Such rocks acquire 
a distinctly platy appearance. 

(d) Xenoliths 

The pseudoleucite borolanites of the Aultivullin vicinity contain xenoliths which 
are particularly noticeable in the north wall of the Quarry. Their presence was 
first recorded by Macgregor & Phemister (1937, p. 45), who ascribed them to an 
early intrusion, or intrusions, of mesocratic melanite-pyroxene-biotite-syenite. 
Xenoliths have not been recognized in other members of the pseudoleucite suite. 

The xenoliths vary in diameter from 0-5 to 50 cm. They are rounded in outline 
(PL 19, 1), sharply bounded, and sometimes have a faint planar structure that is 



FROM THE BORRALAN COMPLEX, SCOTLAND 



299 



enhanced by weathering. There are two types ; the more abundant are dark-green 
in colour, homogeneous, pyroxene-rich rocks, while the others are essentially 
melanocratic borolanite, free of pseudoleucite. 

The pyroxene-bearing xenoliths [1972, P8 (173), (277) & (340)] are equigranular 
rocks of subhedral, sometimes zoned, aegirine-augite of composition Di. 53, Hd. 
13-6, Ac. 33-3 (Tyler & King 1967, analysis Bo 270), and finely twinned microcline 
grains between 0-05 and 0-2 mm (PI. 19, 2). In a few samples the feldspar is more 
variable in size, while the pyroxene is concentrated into clots and streaks and 
varies greatly in its degree of idiomorphism ; it is also more aegirine-rich. Ragged 
biotite when present is pleochroic with a bright-orange and /? — y olive-orange. 
Apatite, pleochroic foxy-brown sphene, carbonate and fluorite are accessory. One 
xenolith has been found which is rich in alkali amphibole [1972, P8 (345)]. The 
amphibole builds prisms up to 0-2 mm in length and is often intergrown with 
deep-green aegirine-augite. The 2V is negative ; birefringence about 0-012 and a 
high dispersion which often prevents complete extinction ; the polarization colours 
are anomalous ; y : c 39 °, a pale yellow-green, £ pale blue-green and y very pale 
blue-green. It is probably a magnesioarfvedsonite (Deer et al. 1963a, p. 370). 
Biotite is restricted to a zone about a centimetre broad against the enclosing 
borolanite. Within the xenolith it is pleochroic from orange to olive-green but 
changes gradually towards the borolanite until it is straw-yellow to dark-green. 

The melanocratic borolanite xenoliths are differentiated from the enclosing 
borolanite by the lack of pseudoleucites and the higher colour index. They con- 
tain large and small garnets, extensively altered to sphene, biotite, irregular and 
prismatic grains of aegirine-augite and apatites which are enclosed poikilitically 
by clear, untwinned plates of potassic feldspar. Dactylotypic intergrowths of 
nepheline and feldspar are widespread, while some zeolitization of the feldspar 
and sericitization of the nepheline has occurred. 



(e) Summary 

The principal mineralogical and textural features of the pseudoleucite rocks can 
be summarized as follows : 







Biotite-magnetite 






Borolanites 


group 


Muscovite group 


Colour index 


30-45 


5-40 


<5 


Grain size 


coarse in west ; 


fine 


coarse and 




coarse/fine in east 




coarse/medium 


Muscovite 


<I0% 


5-40% 


20-80% 


Biotite 


5-15% 


0-20% 


<3% 


Ore 


0-2% 


3-8% 


<3% 


Melanite 


15-25% 


absent 


absent 


Pyroxene/ 


sometimes 


absent 


absent 


hornblende 


present 






Zeolite 


0-15% 


absent 


absent 


Nepheline 


0-25% 


absent 


absent 


Cancrinite 


0-8% 


absent 


absent 



3°o 



PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 



(2) Lower suite : the Aultivullin Quarry boreholes 

The outcrop of the lower suite is confined to the Allt a'Mhuilinn, but it comprises 
the bulk of the rocks encountered in the Aultivullin Quarry boreholes. Of these 
rocks only the vullinite (Shand 1910, p. 406 ; Macgregor & Phemister 1937) and 
granular borolanite of the Gorge Section (Shand 1910, p. 409) have been described 
previously. 

The Robertson Research Company kindly gave slices taken at roughly 5 feet 
(1-52 m) intervals, from all three boreholes. (These rocks have the British Museum 
(Natural History) number 1969, P17.) Text-fig. 4 is based on sections made from 
these slices and shows, schematically, the vertical range of pseudoleucite, and the 
presence, or absence, of pyroxene, hornblende, andradite and melanite with depth. 



Pseudo- Pyrox- Horn- Andra- Mela- 
leucite ene blende dite nire 



10 



20 



30 



T 



Pseudoleucite 
Borolanite 



Pyroxene 
Group 



Granular 
Borolanite 




10 
20 
30 
40 
50 
60 
70 
80 
90 
100 

no 
120 ■ 

40 '30 • 
140 • 
150 • 



160 VV\*V\A/V\^v^\^v^A'\A/VVV\/CaA'vVVV\AA 

Metres Feet 

Depth 

Fig. 4. The vertical range of pseudoleucite and the principal mafic minerals encountered 
in the Aultivullin Quarry boreholes. The four major rock divisions are indicated. Stip- 
pling shows range of feldspathization. 



II 



Hornblende 
Group 



FROM THE BORRALAN COMPLEX, SCOTLAND 301 

The subdivisions which have been erected, according to the changes of mineralogy, 
are shown also on Text-fig. 4. The stippling at deeper levels indicates the range 
of pink syenite veins and a general reddening of the rocks. The pseudoleucite 
rocks, which extend to a depth of 20 feet (6-1 m), are not described in the following 
section as the account of the pseudoleucite rocks already given applies equally to 
those of the boreholes. 



(a) Pyroxene group 

This group is somewhat variable in texture and mineralogy. Two extreme 
types can be identified which grade into each other, and which are usually rep- 
resented in each thin section. One rock is a pyroxene-biotite syenite showing 
simple grain boundaries, fluidal panidimorphic textures and with homogeneous, or 
slightly perthitic, alkali feldspar. The other is an andradite-biotite syenite charac- 
terized by very ragged intergrowths of biotite and garnet, and two distinct feldspar 
phases which also develop complicated interlocking non-equilibrium textures. 
The development of the andradite-bearing rock from the pyroxene-biotite syenite 
can be followed in thin section. Neither type persists for more than one or two 
centimetres before grading into the other. 

The pyroxene-biotite syenites are mesocratic rocks in which the alkali feldspar 
may be optically homogeneous, or a perthite of string-bead type. It varies from 
euhedral to anhedral in form, and may be Carlsbad twinned ; a linear texture is 
sometimes evident and boundaries may be simple, meeting at 120 triple junctions, 
or amoeboid. It is variably turbid and encloses the other minerals poikilitically. 

The pyroxene is a diopside with 0:741-45° and a very pale-green colour, 
sometimes slightly pleochroic to a pale yellow-green. Deeper green rims of diopsidic 
aegirine-augite are sometimes present. It forms euhedral to subhedral prisms up 
to 2 mm long, and much smaller anhedral ' granules ' which commonly build clusters 
in association with magnetite and biotite. The prismatic and granular forms occur 
together in most sections. Biotite with a yellow, £ = y very dark green is less 
abundant than pyroxene and ranges from euhedra to completely ragged flakes. 
It is commonly intergrown with pyroxene and may be replacing it in part. Horn- 
blende has been found in three specimens of the pyroxene group. It builds poikilitic 
crystals of an open type in which segments are often apparently isolated, though 
optically continuous (PI. 21, 4). Feldspar is included in this way, but the over- 
growth of granular pyroxene clusters appears to be replacive (PI. 20, 2), as are 
hornblende rims to individual pyroxenes. Large numbers of small, brown grains 
of sphene included by the hornblende may be a product of replacement of pyroxene, 
as they are rare in the rest of the rock. Pleochroism of the hornblende is a yellow, 
/J very dark greenish-brown, y blue-green ; y : c 32°. 

Accessory minerals include short, stout euhedra of apatite as much as 1 mm in 
length, euhedral to anhedral magnetite and brown pleochroic lozenges of sphene. 

The andradite-biotite syenites show stronger signs of deformation than the 
pyroxene-biotite syenites in the form of a streaky development of mafic minerals, 
a ' flaky ' and finer-grained texture of the feldspars and distorted mica cleavages. 



302 PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 

A high proportion of the feldspar is oligoclase as independent anhedral grains, and 
irregular patchy intergrowths in perthite. In some places the oligoclase forms 
subhedral phenocrysts up to 2 mm long with inclusions of biotite, which are very 
similar to those in the rock vullinite. Biotite and andradite are intimately inter- 
grown and form irregular clusters and streaks, and sometimes pseudomorph pyroxene 
(PL 21, i). The development of biotite and andradite at the expense of pyroxene 
can be followed in thin section. The biotite is pleochroic from apple-green to pale- 
yellow, but develops brown halos around apatite and some sphene crystals, while 
the andradite varies from colourless to pale-brown and occasionally contains cores 
of deeper-brown melanite. The andradite is invariably anhedral and is aniso- 
tropic showing complex sector twinning. The differences in composition of the 
andradite and melanites from pseudoleucite borolanite are apparent from Table I. 

The same accessory minerals are found as in the pyroxene-biotite syenites, but 
with the addition of some carbonate and muscovite. It must be stressed that the 
andradite-biotite syenite is formed from the pyroxene-bearing syenite and that the 
two are intimately mixed on a small scale according to the alteration relationship : 

pyroxene + alkali feldspar + (ore) -» biotite + andradite 
+ oligoclase + (alkali feldspar) 

(b) Granular borolanite 

Apart from the absence of pseudoleucite, the granular borolanites are mineralo- 
gically very similar to the pseudoleucite borolanites except that nepheline has not 
been identified. , There are, however, textural differences. The alkali feldspar 
builds a granular mosaic of simply bounded crystals, not the large poikilitic plates 
of the pseudoleucite borolanites. Oligoclase is patchily distributed, often in vein- 
like areas, and forms complexly intergrown masses in the vicinity of which the 
alkali feldspar becomes very turbid and perthitic textures are evident. Zeolitiz- 
ation of the feldspar is extensive, but unevenly distributed, and cross-cutting, very 
narrow veins of thomsonite occur. Rare angular patches of zeolite, sericite and 
sometimes cancrinite, are possibly after nepheline. 

Green biotite and yellow to brown melanite form very complicated intergrowths 
in which either one may be included in the other. However, the textural evidence 
suggests replacement of biotite by melanite, and nucleation of melanite in biotite. 
Biotite also displays cuspated margins towards feldspar. Melanite is commonly 
moulded around feldspar (PL 20, 3) and apatite, and is rarely euhedral, but forms 
very ragged or amoeboid crystals, in striking contrast to its euhedral habit in the 
pseudoleucite borolanites. Alteration to sphene is sometimes evident, and myriads 
of small brown sphenes are occasionally found in biotite. 

Green diopside occurs in the top few feet of the granular borolanites and, together 
with hornblende, in the bottom few feet. Minor carbonate, muscovite and can- 
crinite are found in some sections, while apatite, which causes brown halos in 
juxtaposed biotite, and ore are invariable accessories. There is often evidence of 
shearing. 



FROM THE BORRALAN COMPLEX, SCOTLAND 303 

(c) Hornblende group 

These rocks, lying beneath the borolanites, and with further thin layers of 
borolanite developed within them, are characterized by the presence of hornblende, 
which is invariably in greater abundance than pyroxene. As in the pyroxene group 
there are essentially two main rock types interleaved, in this case a hornblende- 
pyroxene syenite and an andradite-biotite syenite, of which the latter has formed 
at the expense of the former. 

The hornblende-pyroxene syenites are medium grained with a variable colour 
index up to 50 and either a linear fabric defined by aligned feldspar, hornblende 
and biotite or, more commonly, a granular texture. A layered structure is some- 
times defined by variations of grain size and mineral proportions. Simple inter- 
feldspar grain boundaries and triple junctions are well developed. The feldspar 
is an optically homogeneous alkali feldspar with only rare oligoclase crystals. 
Zeolitization of feldspar is occasionally quite extensive, and dactylotypic inter- 
growths in feldspar of sericite have been found in two specimens, and are very 
probably after nepheline. Hornblende, with y : c 30 and a yellow, /3 very dark- 
green and y bluish -green, forms stout subhedral crystals and complex sieved grains 
which include and are moulded around feldspar. Cores of pyroxene in hornblende 
occur in most specimens (PI. 21, 3) and this texture, together with the rimming of 
clusters of pyroxene grains by hornblende and insinuation of hornblende between 
the pyroxenes, indicates some degree of alteration and replacement. There is a 
complete gradation from rocks in which the evidence suggests that all the hornblende 
has grown at the expense of pyroxene to rocks in which a clean feldspar-hornblende 
fabric with only occasional pyroxene cores suggests primary crystallization of 
hornblende. 

The pyroxene is a pale-green diopside indistinguishable from that of the pyroxene- 
biotite syenites, while the euhedral to subhedral biotite, pleochroic with a yellow, 
jS = y very dark-green, almost opaque, is also the same. Accessory minerals 
include apatite, sphene and magnetite. 

As with the pyroxene group these rocks are affected by a change whereby horn- 
blende and pyroxene alter to andradite and biotite (PI. 21, 2), with a concomitant 
increase in the proportion of oligoclase and perthite. These changes may be 
restricted to small patches, follow very narrow zones, or pervade several centi- 
metres of rock. The feldspar, dominantly oligoclase, is very variable in size, but 
of a finer grain than in the andradite-free parts, and with complex grain boundaries. 
The original alkali feldspar becomes turbid and usually displays exsolution of a 
sodic phase. All degrees of alteration of pyroxene and hornblende to andradite 
and biotite occur, the final product being an andradite-biotite syenite. The 
andradite is invariably anhedral as ragged clusters and streaks, and is usually 
anistropic and sector twinned. Larger grains occasionally have pale-brown cores. 
This relationship is particularly noticeable in the borolanites, which occur inter- 
mittently among the hornblende group (Text-fig. 4), in which brown melanite and 
colourless andradite are intergrown. The secondary biotite, which is intergrown 
with andradite, is of a slightly paler colour than the primary biotite, and is pleo- 
chroic from pale-yellow to apple-green. Accessory minerals are apatite, sphene, 



30 4 PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 

sericite, magnetite and carbonate which is probably partly a product of the alter- 
ation of pyroxene. 

Although there is an increase in the turbidity of the feldspar in the andradite- 
biotite syenites, there is also an alteration of the feldspar related to the reddening 
which is apparent in hand specimen, and which increases with depth. The feld- 
spars in the bottom 10 feet (3-05 m) of the two deepest boreholes are almost opaque 
in places. The only mafic mineral is a green, very fine-grained biotite, with some 
admixed magnetite. However, a section from the lowest level of the deepest 
borehole [1969, P17 (73)] contains unaltered hornblende together with andradite 
and biotite, although the feldspar is very turbid indeed. 

As in the pyroxene group shearing is shown by broken and bent crystals, in more 
extreme cases by mortar structure and ultimately by a very fine-grained, streaky 
rock. 

(3) Lower suite : the Allt a'Mhuilinn section 

(a) Vullinite 

In hand specimen vullinite is a rather fine-grained, sometimes foliated, dark- 
grey rock speckled with feldspar tablets up to 3 mm long. It is built of a mosaic 
of potash feldspar, albite-oligoclase and biotite in which are set larger grains and 
clusters of pyroxene and plagioclase ; hornblende is occasionally important and 
ore, sphene and apatite are plentiful. The potassic feldspar is sometimes perthitic 
and is confined to the groundmass. The albite-oligoclase of the groundmass may 
develop amoeboid grains or a simpler granular fabric, while the phenocrysts have 
sutured margins. Albite, Pericline and Carlsbad twins are present but are usually 
obscured by alteration, which is confined to the cores of grains while the rims are 
quite fresh and untwinned. Euhedral to subhedral biotites are included by the 
plagioclase phenocrysts in which they appear to have grown (PI. 20, 1). Biotite 
is noticeably less abundant in the hornblende-bearing varieties. It forms stout 
books up to 0-5 mm long, which are pleochroic with a yellow and ^ — y deep- 
green, and sometimes defines a strong foliation. Large plagioclases cut across the 
biotite foliation but there is also some deflection of the biotites around them 
(PL 20, 1). The larger pyroxenes, up to 1-5 mm long, tend to be prismatic, but 
smaller aggregated grains are invariably anhedral. They are zoned from a very 
pale-green or colourless core to a pale-green rim ; y : c 45 , but lower at the rim. 
Sphene, ore and biotite are sometimes included by the pyroxene but may also be 
moulded around it. Hornblende is pleochroic with a yellow, fi deep olive-green 
and y deep bluish-green ; 2V( — ) is moderate and y : c 35 . It is poikilitic towards 
feldspar and occasionally to sphene, ore, apatite, biotite and pyroxene, and is only 
found in the vicinity of aplite veins. Anhedral magnetite is included by biotite, 
pyroxene and hornblende but more often is moulded around these minerals, and 
penetrates them along cleavages and cracks. The abundant anhedral sphene is 
pleochroic from yellow to reddish-brown and often rims ore. Apatites are an- 
hedral, except in tiny crystals, while epidote is secondary after plagioclase and also 
forms thin, discontinuous veins. 



FROM THE BORRALAN COMPLEX, SCOTLAND 305 

Shand (1910, p. 407 ; 1938, p. 416) considered vullinite to be a metamorphic 
rock, as did Phemister (Macgregor & Phemister 1937). There is no doubt that its 
textures resemble those of some hornfelses. 

(b) Shonkinite 

This rock is restricted to a layer some 12-15 m long by 3-4 m thick on the 
eastern side of the middle part of the Gorge. It has a well-defined layered struc- 
ture [1972, P8 (289) & (399)] parallel to which are lenses and pods of borolanite 
which are more resistant to weathering. Hand specimens are dark-green in colour 
and have a pronounced foliation picked out by more melanocratic and leucocratic 
layers. Modal analyses give pyroxene 50-60 per cent, zeolite 22-28 per cent, 
alkali feldspar 13-17 per cent, sphene 1-2 per cent, melanite 1 per cent, biotite 
0-3 per cent and minor magnetite. The equidimensional pyroxenes have a charac- 
teristic sugary texture, which is very similar to some of the xenoliths in the pseudo- 
leucite borolanites (PL 19, 3). They vary between 0-05 and o-i mm in diameter, 
are slightly pleochroic with a green, jS slightly paler-green, y greenish-yellow, and 
2V(4-) 6i°. The potassic feldspar occasionally displays microcline cross-hatching 
or Carlsbad twinning. It is extensively zeolitized to fibrous, stellate, straight 
extinguishing clusters (probably thomsonite). Cross-cutting feldspathic veins are 
quite unaffected by the zeolitization. Andradite, melanite and sphene are patchily 
distributed along the foliation, and the melanite is commonly moulded around 
pyroxene and feldspar and forms wormy growths through the rare biotite. 

(c) Borolanite 

Borolanites of the lower suite occur along the full length of the Gorge, but they 
are variable in type. In none of them is pseudoleucite to be found, the pseudo- 
leucite borolanites being confined to levels above the shonkinite. In the upper 
part of the Gorge borolanite forms layers within the shonkinite and the shonkinite 
grades laterally into borolanite. These borolanites are granular rocks, occasion- 
ally with a dimensionally orientated fabric of prismatic feldspars or elongated 
clusters of mafic minerals, and invariably with a greater or lesser proportion of a 
diopsidic pyroxene. The pyroxene is pale-green, often with deeper green rims, 
and may form tiny, fresh grains or larger prisms, up to 1 mm, with rims of biotite 
and melanite. Clusters of green biotite with intergrown melanite often contain 
remnant cores of pyroxene. The biotite-melanite clusters typically have the 
melanite forming worm-like growths through the biotite, and undoubtedly these 
two minerals have replaced earlier pyroxene. The feldspar is dominantly an 
optically homogeneous alkali feldspar, but it may be perthitic. Oligoclase is 
present in widely varying amounts. There is some zeolite present, and sphene, 
apatite and ore are accessories. 

Similar borolanites occur in the middle and lower sections of the Gorge, but 
pyroxene is a rare constituent of these, though the biotite-melanite clusters are 
present. Andradite, which is rarely found in the upper part of the Gorge, is com- 
monly intergrown with melanite in the lower Gorge rocks. 



306 PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 

(d) Andradite-biotite syenites 

The rocks in the lower part of the Gorge are dominantly syenites with andradite 
and biotite as the principal mafic minerals. They have mineralogical and textural 
features in common with some members of the pyroxene group from the boreholes. 
The feldspar varies from a homogeneous granular mosaic of potassic feldspar and 
lesser amounts of oligoclase to a porphyritic texture of heavily exsolved perthites 
up to 2 mm across in a matrix of oligoclase and potassic feldspar averaging some 
o-i mm diameter. The andradite is invariably anhedral, often quite ragged, 
while the biotite, though usually green, is brown in patches and commonly has 
brown halos against orthite or sphene crystals. One specimen [1972, P8 (403)] 
contains 2-3 per cent of hornblende and pyroxene which are being replaced by 
andradite and biotite. Sphene, apatite, orthite and epidote are accessory, and 
cubes of pyrite are very abundant in some specimens. 

(4) Other rock types 

(a) Leucocratic borolanite 

These borolanites, restricted to a line of cliffs north of the Quarry, are light-grey, 
massive rocks in which mafic minerals form only occasional clots and streaks up 
to a centimetre across. The junction with the pseudoleucite borolanites is clear cut 
against the matrix of the pseudoleucite rocks but indistinguishable against the 
pseudoleucites. The leucocratic borolanites are of two types : a group rich in 
nepheline and a group of potassic feldspar-muscovite rocks broadly similar to 
those along the eastern margin of the South-eastern Tract. 

The muscovite-rich rocks consist of unevenly extinguishing potassic feldspar 
plates up to 6 mm in diameter, which are sometimes slightly perthitic and usually 
turbid and untwinned, and include and replace clusters and individual flakes of 
muscovite. Relatively rare biotite is partly altered to muscovite and sporadic 
melanite is much altered to sphene and ore. Cancrinite is common and some 
carbonate is present. The feldspar poikilitically encloses all the other minerals. 
These muscovite-rich borolanites lie adjacent to the pseudoleucite borolanites and 
the contact, as seen under the microscope, merely defines a sudden change in the 
abundance of the mafic minerals. Biotite alters to muscovite at the contact. 

The nepheline-bearing leucocratic borolanites are composed dominantly of 
untwinned potassic feldspar plates up to 8 mm across through which nepheline, 
of the dactylotypic and coarser types, is riddled. The frequent continuity of the 
dactylotypic canals with large, easily recognizable, nepheline grains shows clearly 
that the mineral in the canals is nepheline. Cancrinite forms small veins, patches 
and individual flakes in the nepheline and feldspar, and there is some development 
of zeolite. Biotite and garnet are sparsely distributed, usually in aggregates up 
to a centimetre across, and the larger garnets show the usual alteration to sphene. 
Apatite, ore and very rare aegirine-augite have also been recognized. 

(b) Aegirine-nepheline-analcime borolanite 

An unusual borolanite forms the southernmost of the exposures to the west of 
the Alltan nam Breac [1972, P8 (334)]. Its contact relationships cannot be seen. 



FROM THE BORRALAN COMPLEX, SCOTLAND 307 

It is a homogeneous, grey rock consisting of a fine-grained mosaic of potassic 
feldspar, fresh nepheline and melanite ranging from 0-05 to 0-2 mm in diameter, 
in which are set potassic feldspar porphyroblasts up to 5 mm across and smaller 
ragged biotites and aegirines. The porphyroblastic nature of the feldspar is indi- 
cated by numerous inclusions of all the other minerals and intricately interdigi- 
tating boundaries (PL 18, 4). Between the feldspar and the nepheline there is 
invariably an even, thin, isotropic layer which is probably analcime. The biotite 
is pleochroic with a yellow, j8 = y very dark-green. The aegirine forms stubby 
subhedral prisms and ragged grains which include and are moulded around feld- 
spar ; the pleochroism is from yellow to a deep emerald-green ; a : c is very small. 
A similar rock has been found in an isolated exposure just to the north of the 
road one-third of a mile (540 m) east of Aultivullin [1972, P8 (365)]. This rock is 
abundantly endowed with nepheline and contains considerable pyroxene and anal- 
cime. The subhedral to anhedral pyroxene is a green, unzoned aegirine-augite with 
a : c 58 , and the texture is similar to that of a pseudoleucite borolanite. Melanite 
is zoned from brown to honey-yellow, and the nepheline, which forms nearly a 
quarter of the rock, has a narrow zone of analcime between it and the feldspar. 



(c) Dykes and veins 

Veins are plentiful in the borolanites of the Quarry and among the rocks of the 
Gorge, but are relatively rare over the rest of the South-eastern Tract. They 
comprise an earlier suite of potassic feldspar-nepheline pegmatites, and pegmatites 
rich respectively in melanite, biotite, cancrinite or analcime. These veins are cut 
by a later suite of microcline-microperthite veins and aegirine aplites which are 
comagmatic with the later syenites of the Complex. 

The potassic feldspar-nepheline pegmatites may be sharply cross-cutting or form 
segregations which grade into the adjacent rock. Nepheline is often quite fresh, 
but may be altered or replaced, wholly or partially, by sericite or cancrinite. There 
is invariably a uniformly narrow ' corona ' of analcime between nepheline and 
feldspar. Cancrinite, magnetite, melanite, biotite, albitic plagioclase, sphene and 
orthite are accessory. Zeolitization is often extensive. 

In the Quarry, pegmatites with books of biotite up to 10 cm across are common, 
while masses of melanite garnet as much as 5 cm across also occur. The garnet 
is often concentrated as broad selvages to veins. Cancrinite and analcime peg- 
matites from the Quarry have been described and analysed by Stewart (1941) 
and the exceptional richness in S0 3 ~ radical of the cancrinite led to its classifi- 
cation by Deer et al. (1963b, p. 313) as carbonate vishnevite. The melanite 
analysis given in Table I (anal. A) is from one of these pegmatites. 

The later veins and dykes include a suite of microcline-microperthite veins with 
accessory biotite, albitic plagioclase, magnetite, carbonate, aegirine and andradite 
(probably xenocrystal). Aplitic dykes, which are finely granular rocks but oc- 
casionally porphyritic, may be composed dominantly of microcline-microperthite 
or a mixture of microcline-microperthite and sodic plagioclase. Aegirine, quartz 
and sphene are present. The presence of microcline twinning and the complete 



308 



PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 



absence of zeolitization, together with the cross-cutting relationships, serve to 
distinguish this group from the earlier veins. 

V. ALKALI FELDSPARS 

The potassic feldspars of the muscovite group rocks commonly display microcline 
cross-hatching, which is never found in the borolanite feldspars, although these do 
have uneven extinction patterns. Miyashiro (1957, p. 392) described feldspar 
from borolanite in which the 2 V ranged from 52 ° to 76 , and the obliquity was 
variable. Feldspars have been separated from 40 pseudoleucite rocks of the South- 
eastern Tract and they have been investigated by X-ray diffraction, while the weight 
percentages of Na 2 and K 2 have been determined by wet chemical methods on 
30 of these (Table II). 

Table II 

Structure and composition of South-eastern Tract feldspars 
Borolanites Biotite-magnetite group 



No. 


Feldspar 


Obliquity 


Wt. 


Wt. 


/o 


No. 


Feldspar 


Obliquity 


Wt. 


Wt. 


O' 

/o 




structural 






/o 


% 


Or 




structural 




0/ 

/o 


% 


Or 




type J 






Na 2 


K 2 






type 




Na 2 


K 2 




50 


1 






2-0 


14-4 


83 


161 


3 




0-79 


14-9 


92 


56 


2 






1-7 


13-5 


84 


323 


2 




i-3 


i3-i 


87 


61 


2 





•65 


- 


- 


- 


325 


2 




0-48 


14-7 


95 


84 


2 






0-9 


14-7 


9i 


342 


3 


0-63 


- 


- 


- 


90 


1 






0-87 


14-5 


9i 


350 


2 


0-46 


o-35 


15-9 


97 


114 


1 






1-7 


13-6 


84 


351 


4 


0-74 


115 


152 


89 


116 


1 






1-2 


14-9 


89 


362 


2 


0-49 


2-2 


12-4 


79 


123 


2 






2-2 


I3-I 


80 














285 


2 






0-32 

T • A C 


15-4 

T O • A 


97 
86 




Muscovite group 






290 


I 






1 45 


1 i 4 














329 


2 






0-48 


15-3 


95 


129 


4 


0-79 


o-35 


15-8 


97 


344 


2 






0-24 


16-0 


98 


130 


3 


o-73 


o-33 


15-8 


97 


355 


2 






0-48 


15-9 


96 


131 


4 


o-8i 


0-36 


16-0 


96 


356 


3 





75 


0-67 


15-6 


94 


142 


3 


0-46 


0-58 


153 


94 


363 


1 






0-16 


16-1 


98 


145 


3 


0-69 


- 


- 


- 


378 


2 






0-12 


15-9 


99 


159 


3 




0-32 


i5'4 


97 


ti9i2, 














160 


4 


0-83 


0-52 


15-2 


95 


700 


2 






o-43 


15-4 


97 


352 


4 


0-74 


- 


- 


- 


*7585 


1 






0-18 


16-0 


98 


388 


4 


0-64 


0-64 


15-0 


94 


Alkali analyses by 


C. J. Elliott 


















* Sp 


ecimen given by 


Professor Tilley (see 


Tilley 


1958, Table I, no. i) 










f British Museum (Natural 


History) specimen. 


Described and analysed by Campbell Smith (1909). 


J See text. 

























The 131-131 reflections in the 26 range 29-30^° indicate that there is a complete 
transition from orthoclase to microcline. It has proved practicable to divide the 
feldspars, according to their 131-131 reflections, into four groups which are illus- 
trated in Text-fig. 5. The grouping has not been made on a quantitative basis 



FROM THE BORRALAN COMPLEX, SCOTLAND 



309 





21 Days 




18Do r s 





9Doys 





Unhealed 




30 



29 30" 

2e(CuKa) 



29- 



Fig. 5. Diffractometer traces of feldspars from pseudoleucite rocks (left) and traces of 
heat-treated Madagascar microcline (right). For explanation of groups 1 to 4 see text. 



3io PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 




Fig. 6. Areal distribution of feldspars groups i to 4 over the South-eastern Tract, and 
obliquity values. Filled circles, group 1 ; open circles, group 2 ; open squares, group 
3 ; filled squares, group 4. 



FROM THE BORRALAN COMPLEX, SCOTLAND 



3" 



but a rough estimate of the proportions of orthoclase and microcline can be made 
by comparison with the curves given by Steiger and Hart (1967, Fig. 3) for arti- 
ficial mixtures. Group 1 feldspars have orthoclase > 80 per cent ; group 2, 
80-40 per cent orthoclase ; group 3, 40-20 per cent orthoclase ; group 4, < 20 
per cent orthoclase. Also shown on Text-fig. 5 for comparison are diffractometer 
traces following the transformation of Madagascar microcline to a monoclinic 
form as a result of heating the powdered material in a furnace at 1040 °C for up 
to 21 days. 

The areal distribution of the four structural groups illustrated in Text-fig. 5 are 
plotted in Text-fig. 6 together with obliquity values. There is an overall variation 
from the borolanites containing a feldspar of dominantly orthoclase type in the 
west to the muscovite group rocks which contain microcline with high obliquity 
values and little or no orthoclase component in the east. There appears to be a 
decrease in obliquity values concomitant with an increase in the proportion of 
monoclinic feldspar. 

It is apparent from Table II and Text-fig. 7 that although borolanite feldspars 
are essentially of structural types 1 and 2 and the muscovite group feldspars of 
types 3 and 4, there is little or no chemical difference which can be correlated with 
structural type. 



Type 4 


■ ■■ ■ 









Type 3 


m ■• 








Type 2 


• ••«•• • 




• 


• 


Type 1 

L 


• • 
1 


• 
1 


• • • 


1 



100 



95 



90 85 

/„ Orthoclase 



80 



75 



Fig. 7. Variation of feldspar structural types 1 to 4 with percentage of orthoclase com- 
ponent. Filled circles, borolanites ; open circles, biotite-magnetite group ; filled 
squares, muscovite group. 



Possible mechanisms to explain the variable obliquities of the South-eastern 
Tract feldspars are : (a) variations in the stress field, (b) differences of compo- 
sition of the host rocks, (c) primary cooling patterns, (d) re-heating by a later 
intrusion. 

The first two mechanisms appear to be unlikely : the second two more probable. 
The conversion of microcline to orthoclase in aureoles around intrusive stocks has 



312 PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 

been described by several workers (Doe & Hart 1963 ; Hart 1964 ; Steiger & 
Hart 1967 ; Wright 1967 ; Tilling 1968). The intrusion of the later syenites at 
Borralan could have re-heated the South-eastern Tract rocks converting the feld- 
spars, particularly in the rocks closer to the contact, to orthoclase. In contrast, 
the coincidence in the variation of feldspar type with rock type might suggest 
that the borolanites crystallized at relatively elevated temperatures with the 
production of the high-temperature polymorph orthoclase, while the lower- 
temperature polymorph microcline, was produced in the more easterly rocks of 
the South-eastern Tract by crystallization at lower temperatures. However, the 
wide variation in rock type, the unevenness of the results, and the relatively poor 
exposure over the South-eastern Tract suggest that any interpretation must be 
approached with caution. 



VI. CHEMISTRY 

Twenty-one new rock analyses, together with two analyses taken from the 
literature, of South-eastern Tract rocks are given in Tables III, V, and VI. The 
localities from which the specimens were collected are indicated on Text-fig. 2. 



Table III 

Chemical analyses, norms and modes of rocks of the pseudoleucite suite 

2 C 3D 4 56789 

Si0 2 47-16 48-19 48-52 48-86 49-05 50-46 52-27 54-35 54-90 57-40 

Ti0 2 1-58 1-75 o-68 0-96 1-57 o-6i 0-55 0-51 0-47 0-20 

Al 2 O a i5 - 94 18-52 20-16 19-50 16-50 20-60 18-50 20-85 20-82 22-88 

Fe 2 3 5-34 4-51 2-13 5-01 6-6i i-68 2-22 2-09 2-60 1-36 

FeO 2-60 i-68 2-20 2-14 2-01 3-15 2-62 1-79 1-25 0-36 

MnO 0-19 - 0-12 0-17 0-21 0-18 0-13 0-09 0-09 o-oi 

MgO 1-33 1-12 0-54 0-95 1-39 107 0-87 0-35 0-25 009 

CaO 10-05 10-29 3-65 6-33 10-22 4-97 4"°5 i"93 2-09 003 

Na 2 3-04 3-44 548 3-65 2-43 i-6o 3-18 0-52 0-51 0-47 

K 2 8-95 8-05 io-io 9-56 7-77 10-75 10-72 13-65 13-26 14-66 

H 2 0+ 1-58 3-00 1-90 1-38 2-03 113 1-31 1-32 1-41 155 

H 2 0~ 0-03 0-45 0-09 0-06 0-05 0-07 0-12 0-09 o-oi 0-03 

P 2 O s 0-31 - 0-17 0-12 0-34 0-26 0-21 0-07 0-06 0-06 

C0 2 2-18 2-27 0-16 o-ii 3-05 2-06 1-53 1-71 0-28 

BaO 0-19 - 0-31 0-34 0-25 0-36 0-28 0-25 0-27 0-19 

SrO 0-18 0-31 — 0-37 — 0-30 0-18 0-15 0-03 

S 0-28 — 0-50 — 0-04 o-oi 0-19 - - - 

S0 3 — — - 0-72 - _____ 

CI - - - 0-14 - - - - - 

NiO _____ 003 - 

100-93 ioi-oo 99-13 100-05 100-95 99-98 99-58 99-57 99-85 99-60 

0=S, CI 0-14 0-25 0-03 0-02 0-005 0-09 

100-79 98-88 IOO-02 100-93 99"97 99'49 







FROM THE BORRALAN COMPLEX, SCOTLAND 




3i3 










Table 


III {cont.) 




















Norms 














2 


C 


3 


D 


4 


5 


6 


7 


8 


9 


Q 


o-o 


o-o 


o-o 


o-o 


o-o 


o-o 


o-6 


o-o 


0-98 


o-o 


c 


o-o 


o-o 


o-o 


o-o 


o-o 


4-98 


o-o 


5-22 


5-63 


6-24 


Or 


38-68 


27-03 


48-56 


35-76 


44-95 


63-54 


63-36 


8o-68 


78-37 


86-65 


Ab 


o-o 


o-o 


o-o 


o-o 


o-o 


6-49 


3-40 


1-50 


4-3i 


o-8i 


An 


3-42 


1132 


0-58 


8-59 


11-17 


3-68 


4-55 


o-o 


o-o 


o-o 


Lc 


1115 


i6-ii 


8-73 


16-27 


0-76 


o-o 


o-o 


o-o 


o-o 


o-o 


Ne 


13-93 


1.5-77 


25-12 


16-73 


11-14 


3-82 


12-73 


i-57 


o-o 


1-72 


Di 


7-14 


602 


1-71 


5-10 


7-47 


o-o 


0-96 


o-o 


o-o 


o-o 


Wo 


8-96 


I3-36 


o-o 


6-04 


1129 


o-o 


o-o 


o-o 


o-o 


o-o 


Hy 


o-o 


o-o 


o-o 


o-o 


o-o 


o-o 


o-o 


o-o 


0-32 


o-o 


Ol 


o-o 


o-o 


1-38 


o-o 


o-o 


4'74 


2-95 


115 


o-o 


o-o 


Mt 


4-42 


o-34 


3-09 


4-67 


261 


2-43 


3-22 


303 


2-96 


o-o 


Hm 


2-29 


4-27 


o-o 


1-79 


4-81 


o-o 


o-o 


o-o 


0-56 


136 


11 


300 


3-32 


1-29 


1-82 


2-98 


116 


I 04 


0-97 


0-89 


0-38 


Ap 


073 


o-o 


0-40 


0-28 


o-8o 


o-6i 


049 


016 


0-14 


0-14 


Cc 


4-96 


o-o 


5 -i6 


036 


0-25 


6-94 


4-68 


3-28 


3-59 


009 


H 2 0+ 


1-57 


3 00 


1-89 


i-37 


2-02 


I-I2 


1-30 


1-32 


141 


i-55 


H 8 0- 


003 


o-45 


0-09 


006 


OO5 


0-07 


0-12 


009 


001 


0-03 


Others 


065 


o-o 


112 


1-20 


o-66 


0-40 


0-77 


o-43 


042 


o-8i 


Total 100-93 


IOI-OO 


99- 13 


IOOO5 


IOO-95 


99-98 


99-58 


9957 


99-85 


99-60 










Modes 












K-feldspar 






- 


39 


37-4 




55-4 


52-8 


52-0 


6o-8 


K-feld + nepheline 




55-i 




- 




- 


- 


- 


- 


Nepheline 






- 


19 


- 




- 


- 


- 


- 


Dactylotyp 


ic 




















nepheline 


4- feldspar 


- 


16* 


J 5-4 




- 


- 


- 


- 


Plagioclase 






- 




- 




o-3 


- 


- 


01 


Melanite 






- 


15 


18-3 




5-5 


- 


— 


— 


Biotite 






17-6 


11 


n-6 




27-9 


IOI 


11 


- 


Muscovite 






9-2 




23 




P 


27-5 


37-i 


38-7 


Zeolite 






- 




12-3 




- 


— 


- 


- 


Carbonate 






6-5 




- 




70 


3° 


4-8 


— 


Sphene 






5-8 




2-4 




2-7 


- 


P 


- 


Apatite 






P 




o-3 




0-2 


- 


- 


- 


Fluorite 






0-9 




- 




o-8 


- 


- 


- 


Ore 






4-9 




- 




0-2 


6-5 


5-o 


01 


* Includes cancrinite (Tilley 


1958, p. 


158). 














2. Pseudoleucite borolanite 


[1972, 1 


P8 (356)] 














C. Pseudoleucite borolanite 


(Campbell Smith 


1909, p. 154) 










3. Pseudoleucite borolanite 


[1972, ] 


P8 (37o)] 














D. Pseudoleucite borolanite 


(Tilley 


1958, p. 1 


57) 












4. Pseudoleucite borolanite 


[1972, ] 


P8 (290)] 














5. Pseudoleucite biotite-ma 


.gnetite rock [197: 


z, P8 (362)] 












6. Pseudoleucite biotite-ma 


.gnetite rock [1972, P8 (364)] 












7. Pseudoleucite biotite-ma 


gnetite rock [197: 


2- P8 (350)] 













Pseudoleucite biotite-magnetite rock [1972, P8 (331)] 
Pseudoleucite muscovite rock [1972, P8 (131)] 

For localities of analysed rocks see Text-fig. 2. 

Analyst: C. J. Elliott 



3^4 



PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 



3 
O 
u 
tuO 



> 

o 
o 



+ 

o 



Os o O t|- s£> t^ O 

u-i so r-~ £<« r-» £*. ob 

Os Os Os Os Os Os Os 



0> MJ Qi + i-O 
io f fi en to co n 

6 6 6 6 6 6 6 



O vO 
00 SO 



O so SO N I~» 

r-» so o to r-» 



CO Tf (N * tJ- t)- f| 



(N M lO Os Os N 

t «1 * IO N (O N 

M M M IH >H M CO 



Os 



o 
1- 

6 



00 



W 
►J 

m 
< 



o 

3 

<u 
"o 

3 
cd 
en 

P-l 

V 



M-l 

o 



'J 

o 



o 



c 



ft 
3 
O 
H 
be 



C 
be 

Ctj 



! 



PQ 



o 
o 






O 



o 



+ 

O 

n 



hNNOOOOihcnooco 

f^ob <n n t> Jo ui inb i 

N N00 00 W Oi IJi O Ol fji 



OOOOooOOsOOOsOwN 
M N H ^O SO 00 f^ SO SO LO 



CO 


N N 


M M 


o 


o 


o o 







01 00 


lO SO 


o 


CO 


lO so 

O N 




O 

M 


Os O 


O H 


m 


m 


oo oo 


PO 



Tj-NoONiOMi-itNi-iO 
so ^ N sO N irjsD io fO iO 
ro ro fO to (O f) h rococo 



Os 

00 
00 






O 

o 






3 

"o 

O 

m 



o 

o 






o 

Csl 



o 



+ 



M 



00 w -t Oi n so O O N fOOO so 



■*■ o 

so t^ 



P) CS * + so© sO CO OsOO 

r^t^r^t^r^r^r^c^t^oo 



00 Tf m CO sO 

■+ i-O sO h 



■*■ ■<*■ O ION O 

o r^ so ■*■ >o w 



lO CO CO CO CO CO <N N N N IN M 



O VO sO 


so 


Os so r> to 


r^ 


Os 00 


lO 




N 




i-i O SO 


t> 


O Os * N 


r> 


O CN 


■<J- 




00 


> 


O 00 Os 

M 


C> 


OSOO CO CO 


t^ 


O 00 


00 


CD 


00 

en 
O 

3 


CO 

>s 














OX) 
cri 


3 














u 


trt 


n 


O 


"*■ 


■*■ sO SO so 


o 


Os CO 


SO 


> 

< 


> 


<! 


t^ * +- 


H 


Tj- SO M 00 


Os 


CO tN 


in 






CO 


H 


CO CO M N 


M 


CO M 


CO 







m 



to 
o 



S^ 



■* 




If) 




M 




(i 




Os 









OS 

H 


io 




H 


,Cj 




+J 


Ch 


fl 


» 


en 


3G 
u-s 


~ 


OS 


a) 




,0. 




&•>> 


fi 


O-i 


cS 


H 


UH 



FROM THE BORRALAN COMPLEX, SCOTLAND 



315 









Table 


V 










Chemical 


analyses and 


norms of Aultivullin Quarry borehole rocks 






10 


11 


12 


13 


14 


15 


16 


Si0 2 


54-59 


5I-56 


48-36 


48-96 


54-4° 


51-27 


50-84 


Ti0 2 


o-68 


o-68 


0-87 


0-89 


0-78 


0-96 


0-21 


A1 2 3 


17-04 


16-07 


17-82 


15-80 


17-60 


15-81 


20-43 


Fe 2 3 


2-80 


3-95 


4-97 


5-12 


2-39 


3-06 


2-19 


FeO 


3-49 


2-56 


2-46 


3-06 


4-12 


5-24 


6-05 


MnO 


0-14 


0-17 


0-23 


031 


013 


0-14 


0-07 


MgO 


3-53 


3-82 


2-36 


2-89 


2-48 


3-54 


2-09 


CaO 


5-7o 


8-o8 


7-92 


7-85 


6-03 


6-69 


i-33 


Na 2 


5-3 


4-i 


3-' 


2-1 


4-7 


3-o 


I-O 


K 2 


4-0 


5-7 


7-7 


8-6 


4-7 


6-3 


9-9 


H 2 0+ 


1-05 


1-79 


1-84 


2-32 


1-02 


0-74 


3-20 


H 2 0- 


0-28 


0-32 


0-52 


o-43 


0-16 


0-24 


0-50 


p 2 o 5 


o-39 


o-45 


o-6o 


0-63 


o-53 


0-78 


0-09 


co 2 


0-78 


o-86 


0-58 


0-71 


o-6i 


I-IO 


i-55 


BaO 


0-26 


030 


0-29 


0-26 


0-23 


0-26 


0-22 


SrO 


0-32 


0-36 


o-54 


050 


o-33 


0-26 


OI3 


Total 


100-35 


100-77 


100-16 

Norms 


100-43 


IOO-2I 


99-39 


99-8o 


Q 


o-o 


o-o 


o-o 


o-o 


o-o 


o-o 


0-69 


c 


o-o 


o-o 


o-o 


o-o 


o-o 


o-o 


8-07 


Or 


23-64 


33-69 


45-15 


46-62 


27-78 


37-24 


58-5r 


Ab 


38-67 


16-58 


o-o 


o-o 


34-22 


20-38 


8-46 


An 


10-90 


8-6i 


n-97 


8-29 


1305 


11-07 


o-o 


Lc 


o-o 


o-o 


0-28 


33° 


o-o 


o-o 


o-o 


Ne 


3'37 


9-81 


1421 


9-62 


301 


2-71 


o-o 


Di 


7-95 


i8-n 


12-68 


16-46 


7-82 


8-22 


o-o 


Wo 


o-o 


o-o 


144 


o-45 


o-o 


o-o 


o-o 


Hy 


o-o 


o-o 


o-o 


o-o 


o-o 


o-o 


12-92 


Ol 


591 


1-19 


o-o 


o-o 


5'°4 


7-71 


o-o 


Mt 


4-06 


5-73 


6-i6 


7-42 


3-47 


4-44 


3-i8 


Hm 


o-o 


o-o 


0-72 


o-o 


o-o 


o-o 


o-o 


11 


1-29 


1-29 


1-65 


I 69 


1-48 


1-82 


0-40 


Ap 


092 


1-06 


1-42 


1-49 


125 


1-84 


0-21 


Cc 


1-77 


1 96 


1-32 


1-62 


1-39 


2-50 


2-16 


H 2 0+ 


103 


1-77 


1-82 


2-30 


I-OO 


0-71 


3-20 


H 2 0~ 


0-28 


0-32 


0-52 


o-43 


0-16 


0-24 


0-50 


Others 


0-72 


o-8i 


0-98 


1-04 


0-74 


0-67 


I- 5 6 



Total 



100-49 



100-92 



10031 



100-71 



100-39 



99-54 



99-86 



10. Pyroxene-biotite-andradite syenite (pyroxene group) [1969, P17 (48)]. Borehole i, at 
depth 25 feet (7-62 m) 

11. Pyroxene-biotite-andradite syenite (pyroxene group) [1969, P17 (35)]. Borehole 2, at 
depth 40 feet (12-19 m) 

12. Granular borolanite [1969, P17 (39)]. Borehole 2, at depth 65 feet (19-81 m) 

13. Granular borolanite [1969, P17 (59)]. Borehole 1, at depth 80 feet (24-38 m) 

14. Hornblende-pyroxene syenite (hornblende group) [1969, P17 (66)]. Borehole 1, at depth 
120 feet (36-57 m) 

15. Hornblende-pyroxene syenite (hornblende group) [1969, Pi 7 (24)]. Borehole iA, at 
depth 136 feet (41-45 m) 

16. Biotite syenite (hornblende group) [1969, Pi 7 (72)]. Borehole 1, at depth 151 feet 
(46-02 m) 

Analyst : V. K. Din 



316 PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 

Table VI 

Chemical analyses, modes and norms of vullinite, shonkinite, a pyroxene-microcline xenolith, 
andradite borolanite, leucocratic borolanite and aegirine-nepheline-analcime borolanite 





17 


18 


19 


20 


21 


22 


Si0 2 


55-io 


48-38 


5 8-6l 


53-6o 


52-07 


50-70 


TiO a 


077 


0-64 


0-65 


0-71 


0-30 


o-53 


A1 2 3 


16-66 


I3-7I 


11-26 


18-14 


23-79 


22-69 


Fe 2 3 


4-50 


1-79 


2-90 


4-52 


0-90 


3-16 


FeO 


3-29 


3-23 


2-73 


1-08 


I-OI 


1-50 


MnO 


0-17 


0-18 


o-io 


0-12 


0-20 


0-07 


MgO 


2-50 


6-37 


4-n 


I-OI 


o-oo 


o-oo 


CaO 


7-08 


17-23 


8-20 


4-35 


2-13 


3-97 


Na 2 


5-o9 


2-00 


1-65 


2-0 


5-69 


4-74 


K 2 


3-99 


2-IO 


9-00 


IO-O 


II-II 


9-85 


H 2 0+ 


o-68 


3^7 


0-22 


1-47 


i-95 


201 


H 2 0- 


0-07 


°-34 


0-08 


o-37 


013 


0-22 


p 2 o 5 


0-69 


0-16 


0-17 


0-23 


0-16 


0-29 


co 2 


0-09 


0-08 


OIO 


115 


0-40 


0-20 


S 


- 


0-04 


- 


- 


- 


- 


BaO 


- 


o-45 


0-02 


o-54 


- 


- 


SrO 


- 


- 


- 


0-40 


— 


- 


NiO 


- 


0-04 


0-08 


0-09 


- 


- 




IOO-68 


100-41 


99-88 


99-78 


99-84 


99-93 


OsS 




0-02 











100-39 



Norms 





17 


18 


19 


20 


21 


22 


c 


o-o 


o-o 


o-o 


o-o 


o-o 


o-o 


Or 


23-58 


12-41 


53-19 


59-10 


43-94 


47-74 


Ab 


36-83 


5" 


7-68 


n-68 


o-o 


o-o 


An 


10-83 


22-23 


o-o 


10-99 


6-56 


n-55 


Lc 


o-o 


o-o 


o-o 


o-o 


17-04 


8-21 


Ne 


3-38 


6-40 


0-07 


2-84 


26-08 


21-72 


Ac 


o-o 


o-o 


5-42 


o-o 


o-o 


o-o 


Di 


15-27 


4J-23 


28-24 


1-42 


0-38 


o-o 


Wo 


o-o 


4-12 


i-53 


o-o 


o-o 


2-08 


Ol 


0-25 


o-o 


o-o 


1-30 


o-6o 


o-o 


Mt 


6-52 


2-60 


1-49 


i-8i 


1-30 


3-53 


Hm 


o-o 


o-o 


o-o 


3*27 


o-o 


o-73 


11 


1-46 


1-22 


1-23 


i-35 


o-57 


I-OI 


Ap 


1-63 


0-38 


0-40 


o-54 


0-38 


o-68 


Cc 


0-20 


0-18 


0-23 


2-6l 


0-91 


o-45 


H a O+ 


0-65 


3-66 


0-21 


1-46 


1-94 


2-00 


H 2 0- 


0-07 


o-34 


0-08 


o-37 


0-13 


0-22 


Others 


o-o 


o-53 


o-io 


1-03 


o-o 


o-o 



Total IOO-68 100-41 99-88 99-78 99-84 99 - 93 



FROM THE BORRALAN COMPLEX, SCOTLAND 



317 



Alkali feldspar 27-4 

Plagioclase 29-0 
Zeolite 
Nepheline 
Nepheline 

(dactylotypic) 
Cancrinite 

Analcime - 

Melanite - 

Biotite 22-6 

Pyroxene 15-6 

Muscovite o-6 

Sphene i-6 

Apatite 0-9 

Ore 23 

Carbonate - 

Fluorite - 



Table VI (cont.) 
Modes 
13-9 59-4 

22-3 



i-3 

0-4 
6o-o 

i-o 

i-i 



392 

0-9 
o-5 



66-3 
1-7 

4-5 



13-1* 
7-9 
o-i 

1-2 
P 

4-6 
o-i 



49-3 

14-4 

8-4 

I2-I 

3-o 

4-6 

5-5 
P 

3-3 
o-8 
01 
0-4 
o-i 



57-i 
P 
P 

22-8 



i-8 

8-4 
4-8 

i-5 

P 

o-i 

3-5 
P 



17. Vullinite [1972, P8 (67)] 

18. Shonkinite [1972, P8 (399)] 

19. Pyroxene-microcline xenolith from pseudoleucite borolanite in Quarry [1972, P8 (277)] 

20. Andradite borolanite [1972, P8 (400)] 

21. Leucocratic borolanite [1972, P8 (378)] 

22. Aegirine-nepheline-analcime borolanite [1972, P8 (334)] 

For location of specimens see Text-fig. 2 

Analyses 17, 21 and 22 by Mrs J. Banham ; 18 and 19 by C. J. Elliott ; 20 by V. K. Din 
* Includes approximately an equal amount of andradite. 



The pseudoleucite series is characterized by very high potash to soda ratios - 
commonly greater than 10 to 1, and often greater than 20 to 1 - while magnesia 
is extremely low, usually less than titania. The values for silica increase from the 
borolanites through the biotite-magnetite group to the muscovite group so that 
Harker diagrams prove useful in reflecting chemical changes with height in the 
layered complex. Upwards there is an increase in silica, alumina and potash 
while iron, lime, titania and soda increase downwards. It is noteworthy that in 
terms of a Harker plot (Text-fig. 8) the biotite-magnetite and muscovite group 
rocks define relatively smooth trends, but that the borolanites show much more 
scatter. The lower suite, however, has a totally different distribution from the 
pseudoleucite rocks in terms of a Harker plot and is not shown on Text-fig. 8. 

In addition to the whole rock analyses alkali determinations only have been 
made on a further 19 pseudoleucite rocks (Table IV). Although there is some 
variation, particularly in the intermediate biotite-magnetite group, the changes in 
the content of K 2 and Na 2 and the ratio of these two oxides varies consistently 
through the series culminating in the average values of 13*8 per cent K 2 and 
0-40 per cent Na 2 in the muscovite group. These very high potash to soda ratios 



3i8 



PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 



8 

7 

6 

Total 5 
iron 

as 4 
FeO 

3 
2 

10 
9 
8 
7 
6 
5 
4 
3 
2 



CoO 



MgO 1 

2 

Ti0 2 1 





1 • 


1 — I 1 1 1 1 1 1 


• 






"" 


• 




- 


• 








© -_ © 




• 








"-J* ® 












o"- 


• 


• • 


- 
















• 








N. 






©•- 


_ 




N© 




• 


s 


— 




v. — 






x _ 








• 


- • 






• • 


~ 9 —-9 




• 


"*-©----_„ 


• 


• • 




- 


• 


— 




• 

1 1 


i i « i i -1--1-0 . 



1 - 



-i — i — i — i — i — i — i — i — i r 



e a - - - o 



3 No 2 

2 

1 



S y- 



u 

- 13 

- 12 

- 11 K 2 
10 
9 



© © 



22 
21 
20 
19 Al 2 3 



18 

17 
- 16 



I I I 



J L 



J L 



47 48 49 50 51 52 53 54 55 56 57 
Si0 2 (Wt%l 



48 49 50 51 52 53 54 55 56 57 
Si0 2 (Wt%) 



Fig. 8. Harker diagram of South-eastern Tract pseudoleucite rocks. Filled circles, boro- 
lanites ; half-filled circles, biotite-magnetite group ; open circle, muscovite group. 



are extremely rare, and comparable rocks usually have much higher magnesia 
values, in contrast to the extremely low magnesia values of the Borralan suite. 
The volcanics of the Leucite Hills, Wyoming (Carmichael 1967), range up to 12-66 
per cent K 2 and 0-94 per cent Na 2 0, but MgO is usually in excess of 6 per cent. 
Similarly the South-west Uganda volcanics (Holmes & Harwood 1937) have higher 
MgO values, as do the lavas of the Roman alkaline province (Washington 1906), 
the Kimberley province of Australia (Wade & Prider 1940 ; Prider i960), and the 
leucite suite of Java (Iddings & Morley 1915). The pseudoleucite tinguaite of 
Tzu Chin Shan, China (Yagi 1954), contains 12-64 P er cen t K 2 0, 5-43 per cent 
Na 2 and 0-03 per cent MgO, which is comparable to the muscovite group and 
some of the biotite-magnetite group rocks, though rather higher in Na 2 0. The 
borolanites with their less extreme compositions can be matched from a number 
of localities. 



FROM THE BORRALAN COMPLEX, SCOTLAND 



319 



Text-fig. 9 emphasizes the strong reciprocal relationship of soda and potash in 
South-eastern Tract rocks. Coombs and Wilkinson (1969, Figs. 8 & 9) on similar 
diagrams give the trends for some 20 provinces and separate intrusions and in all 
of these soda and potash rise sympathetically, though the Shonkin Sag intrusion, 
over part of its range, shows some decrease of soda against potash. Even highly 
potassic provinces such as the Roman Comagmatic region, South-west Uganda 
and Java show ' normal ' trends (Text. -fig. 9, inset diagram), the only exception 
encountered being the Kimberley province, Australia (Wade & Prider 1940 ; 
Prider i960). The mechanism which determined the well-defined Borralan trend 
is not, therefore, usually operative in the majority of igneous provinces. 



Na 2 

(Wt%) 



5 _ 
4 _ 
3 
2 
1 



A 



Na-,0 



J I I I L 




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 

K 2 (Wt%) 

Fig. 9. Plot of soda against potash for pseudoleucite and lower suite rocks. Filled 
circles, pseudoleucite borolanites ; half-filled circles, biotite-magnetite group ; open 
circles, muscovite group ; open squares, pyroxene group ; half-filled squares, horn- 
blende group ; filled squares, borolanites of lower suite ; open triangles, vullinite and 
pyroxene-microcline xenolith ; filled triangles, andradite borolanite. Inset diagram 
shows trends of soda against potash for (a) southwest Uganda volcanic province, 
(b) Java volcanic province and (c) Roman comagmatic region. 



In Text-fig. 10 the pseudoleucite rocks are shown plotted according to their 
normative Qz - Ne - Ks, normative anorthite being relatively small in these 
rocks. They he along a path between potassic feldspar and the nepheline compo- 
sitions defined by Morozewicz (1930) and Buerger et al. (1947). Also plotted are 
the nepheline syenite and its constituent nepheline and feldspar from near Loyne 
at the northwest end of the Borralan Complex, described by Tilley (1956, p. 409, 
Table 4), and comprising his ' juvet type ', the most potassic of the low tem- 
perature nepheline syenite assemblages. 

Analyses were made of pseudoleucites hand picked from three of the analysed 
rocks and these, together with an analysis of a borolanite pseudoleucite given by 
Shand (1906), are given in Table VIII. The compositions were determined of two 



320 



PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 



nephelines (Table VII) taken from a leucocratic borolanite and an aegirine-augite- 
analcime borolanite which crop out among the pseudoleucite rocks. The pseudo- 
leucite rocks themselves were not chosen, because of difficulty in separating the 
nepheline. Partial analyses of feldspars, obtained by electron probe, are given 
in Table IX, and alkali determinations only of feldspars in Table II. The nephe- 
lines and pseudoleucites are plotted in Text-fig. 10, and in Text-fig. n in part 
of the Qz - Ne - Ks system pseudoleucites and feldspars are joined by tie lines 
to their respective rocks. The variation in composition of pseudoleucites and 
feldspars in sympathy with changes in rock composition is apparent. The pseudo- 
leucites are invariably more sodic than their host rocks. Pseudoleucite number 27 
(Text-fig. 11) appears to be unique among analysed pseudoleucites in the com- 
bination of high potash and low soda, and its position in the Qz - Ne - Ks system 
close to the orthoclase point. 





y 

Ab / — 


© 




^\Oz 






3 


t*<^F>- 






\24. 






/ 


© \ 


/© 




/ ® \ 



Ne Ks 

Fig. 10. Plot of pseudoleucite rocks, pseudoleucites and nephelines in the system Si0 2 
(Qz)-NaAlSi0 4 (Ne)-KAlSi0 4 (Ks). Filled circles, borolanites ; half-filled circles, 
biotite-magnetite group rocks ; open circles, muscovite group rocks ; open triangles, 
pseudoleucites ; filled squares, nephelines. E— *— G, nepheline-rock-feldspar of a 
Borralan nepheline syenite described by Tilley (1956, Table 4). Nephelines M and B 
are the compositions of Morozewicz (1930) and Buerger et al. (1947) respectively. Tie 
lines join pseudoleucites to their host rocks. Field boundaries in the dry system are 
taken from Schairer (1957). Extent of soda-leucite solid solution indicated (Fudali 
1963). Stability fields of leucite (L), feldspar (F), nepheline (N), carnegeite (C) and 
kalsilite (K) are shown. 



Text-fig. 12 is part of a von Wolff diagram which illustrates the generally greater 
enrichment in the ' M ' components (MgO, CaO, FeO, MnO) of the borolanites 
compared with the biotite-magnetite and muscovite groups. In terms of this 
diagram there is a trend in the borolanites away from the leucite point, while the 
biotite-magnetite and muscovite group rocks define another trend towards the 
feldspar point. 



FROM THE BORRALAN COMPLEX, SCOTLAND 



321 




Fig. 11. Part of the system Qz - Ne - Ks showing distribution of the pseudoleucite suite, 
and coexisting pseudoleucites and feldspars. Symbols for rocks and pseudoleucites as 
for Text-fig. 10. Lozenges, feldspar compositions. *, nepheline syenite (Tilley 1956) 
as on Text-fig. 10. Numbers and letters refer to analyses in Tables 2, 3, 8 and 9. 





120 



100 



80 



40 



Fig. 12. von Wolff diagram including all analysed South-eastern Tract rocks. Filled 
circles, pseudoleucite borolanites ; half-filled circles, biotite-magnetite group ; open 
circle, muscovite group ; open squares, pyroxene group ; half-filled squares, horn- 
blende group ; filled squares, borolanites ; open triangles, vullinite, shonkinite, and 
pyroxene-microcline xenolith ; filled triangles, andradite, leucocratic, and nepheline- 
aegirine-analcime borolanites. 



322 



PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 



Si0 2 

TiO a 

A1 2 3 

Fe 2 3 

MnO 

MgO 

CaO 

Na 2 

K 2 

H 2 0+ 



H 2 0- 



Total 

23- 

24. 
E. 



23 
41-07 
< 0-03 
36-20 

0-20 



14-46 
7-65 



99-58 



Table VII 

Partial analyses of nephelines 



24 
43-71 
< 0-03 

33-98 
o-37 



14-50 
6-i6 

> 1-30 



E 

41-06 

nil 

33-93 
0-83 
nil 
nil 
019 

15-92 
7-14 
0-74 

nil 



Ne 
Kp 
An 



IOO-02 



99-8 1 



23 


24 


3-4 


8-2 


69-6 


70-0 


27-0 


21-8 



E 

0-7 

73-8 

24-4 
i-i 



Nepheline from leucocratic borolanite, cliffs north of Quarry at Aultivullin (B227). 

Analyst : C. J. Elliott 

Nepheline from aegirine-augite-analcime borolanite, just north of road, ^ mile east of 

Aultivullin [1972, P8 (365)]. Analyst : C. J. Elliott 

Nepheline from nepheline syenite, £ mile southeast of Loyne-Borralan Complex (Tilley 

1956, Table 4) 



Si0 2 

Ti0 2 

A1 2 3 

Fe 2 3 

MgO 

CaO 

Na 2 

K 2 

H 2 (total) 

CO, 



25 

55-67 
0-18 

26-00 

1-44 

0-23 

2-45 

4-47 
9-68 



26 

51-25 
0-23 

22-82 

i-39 
0-06 

3-05 

5-88 

10-40 

2-73 
i-6o 



Table VIII 

Chemical analyses of pseudoleucites 



F 
56-26 

21-93 
0-67 

1-46 

4-95 

10-63 

4-16 



27 

55-83 

0-05 

25-00 

1-65 
o-ii 
0-74 
1-40 
12-42 
2-89 
0-07 





25 


26 


F 


27 


Qz 


35-6 


28-3 


34'i 


42-4 


Ne 


24-9 


31-2 


25-6 


7-7 


Kp 


39-5 


4°-5 


4°-3 


49-9 



Total ioo-i2 99-41 100-06 100-16 

25. Pseudoleucite from pseudoleucite borolanite [1972, P8 (290)] 

26. Pseudoleucite from pseudoleucite borolanite [1972, P8 (356)] 

F. Pseudoleucite from pseudoleucite borolanite (Shand 1906, p. 441) 

27. Pseudoleucite from pseudoleucite biotite-magnetite group rock [1972, P8 (350)]. 



Analyst : C. J. Elliott 



FROM THE BORRALAN COMPLEX, SCOTLAND 323 

Table IX 





Analyses of alkali 


feldspars 






28 


29 


G 


Si0 2 


63-83 


62-68 


63-83 


Ti0 2 


- 


- 


nil 


A1 2 3 


18-24 


18-78 


18-72 


Fe 2 3 


"- 


- 


0-26 


MnO 


- 


- 


nil 


MgO 


- 


- 


nil 


CaO 


- 


- 


nil 


Na 2 


0-56 


I- 45 


0-20 


K 2 


16-48 


15-26 


16-60 


H 2 0+ 


- 


- 


0-07 


H 2 0- 


- 


- 


nil 


p 2 o 5 


- 


- 


(O-II) 



Total 99-n 98-17 99-79 

28. Potassic feldspar [1972, P8 (356)] 

29. Potassic feldspar [1972, P8 (290)] 

G. Feldspar from nepheline syenite, J mile southeast of Loyne-Borralan Complex (Tilley 
1956, Table 4) 

Analyses 28 and 29 done on electron probe by R. F. Symes 



The analyses of rocks from the boreholes (Table V) must be treated with some 
caution for two reasons. Firstly, these rocks are extremely heterogeneous so that 
it was not possible to separate pure pyroxene-biotite syenite from the replacing 
andradite-bearing syenite. Secondly, the slices available for crushing were small. 
The two borolanites from the boreholes (Table V, anals. 12 & 13) prove to be 
closely comparable with the pseudoleucite borolanites, the only significant differ- 
ence being their higher magnesia values. The analyses of pyroxene and horn- 
blende group rocks are rather different from those of the pseudoleucite suite, notably 
in the higher soda to potash ratios which result in normative albite varying from 
8 per cent to 38 per cent, whereas it is negligible in the pseudoleucite suite. Some- 
what higher magnesia and iron values tend to separate the borehole rocks from the 
pseudoleucite suite in terms of the von Wolff diagram (Text-fig. 12), while the 
borehole borolanites plot between the two. There is a very marked difference 
between the two suites in terms of a plot of MgO against (FeO + Fe 2 3 ) (Text- 
fig. 14). The pseudoleucite suite defines a marked differentiation series low in 
magnesia, quite distinct from the more magnesia-rich lower suite. 

The close chemical coincidence of the higher level pyroxene group syenite 
(Table V, anal. 10) and the vullinite (Table VI, anal. 17) is apparent (Text-fig. 13), 
and confirms the correlation of these rocks made on petrographic grounds. While 
pyroxene syenite number 10 (Table V) is strongly altered to an andradite-rich 
syenite and contains an abundance of albite, which usually accompanies this alter- 
ation, the other analysed member of the pyroxene group (Table V, anal. 11) is 
more undersaturated, which is confirmed by the occurrence of altered nepheline 



324 



PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 




Fig. 13. Plot of all South-eastern Tract rocks in the system Qz-Ne-Ks. Symbols as on 
Text-fig. 12. Numbers refer to analyses of Tables 5 and 6. Field of pseudoleucite 
suite outlined. Field of leucite (L) etc. as on Text-fig. 10. 



MgO 
(Wt%) 




1 2 3 4 5 6 

FeO + Fe 2 Q 3 (Wt%) 

Fig. 14. Plot of MgO against (FeO + Fe 2 3 ) (weight per cent) for all South-eastern Tract 
rocks together with ledmorites of the Borralan Complex, and ledmorite dykes of the 
Assynt area. Symbols as on Text-fig. 12 ; ledmorites, crosses. 

in thin section, and is only slightly changed towards an andradite syenite. The 
three analysed rocks representative of the hornblende group (Table V, anals. 14-16) 
show little variation in terms of silica saturation, but considerable variation in the 
ratio of soda to potash (Text-fig. 13). They do not differ significantly from the 
pyroxene group in terms of lime, magnesia and iron (Text-fig. 12). 



FROM THE BORRALAN COMPLEX, SCOTLAND 325 

The pyroxene-microcline xenolith (Table VI, anal. 19) has a high potash to soda 
ratio, which is to be expected as it was enclosed in borolanite, and hence lies amongst 
the pseudoleucite series in terms of Text-fig. 13. The shonkinite (Table VI, anal. 
18) because of its high colour index is low in alkalis but, significantly, contains 
nearly as much soda as potash. These two rocks are the most mafic encountered 
in the South-eastern Tract and hence are enriched in ' M ' components in terms 
of Text-fig. 12. The leucocratic borolanite and aegirine-nepheline-analcime 
borolanite (Table VI, anals. 21 & 22) prove chemically to be very similar to the 
pseudoleucite borolanites (Text-fig. 13), although the leucocratic rock is inevitably 
lower in total iron, lime and magnesia. 

Very high strontium and barium values have been found in all South-eastern 
Tract rocks (Tables III, V and VI). 



VII. DISCUSSION 

There is strong evidence that the pseudoleucite suite of rocks was emplaced 
after the lower suite. This conclusion is drawn from the presence of xenoliths 
within the pseudoleucite borolanites which can be matched with the shonkinites 
of the lower suite ; the layers of borolanite within the shonkinite, which are inter- 
preted on textural grounds as being metasomatic ; and the similar replacement 
features revealed by thin sections of the borolanites from the boreholes. The 
contact between the two suites as seen in the Gorge is apparently sharp, but ob- 
scured to some extent by shearing. However, there is no sign at all of chilling, 
which suggests intrusion of the pseudoleucite magma into rocks which were already 
at relatively elevated temperatures. 

There are three principal problems posed by these rocks : firstly, what mech- 
anism was responsible for the variation within the pseudoleucite suite ; secondly, 
how were the rocks of the lower suite generated, and how are they related genetically 
to the pseudoleucite suite ; and thirdly, how do these rocks relate to the igneous 
history of the Borralan Complex as a whole? 

(1) The pseudoleucite suite 

No intrusive contacts have been recognized among the pseudoleucite rocks, and 
this, together with the gradual, yet consistent, mineralogical and chemical changes 
across the South-eastern Tract suggest that in situ differentiation was the prin- 
cipal mechanism of rock diversification. The marked concentration of iron, 
calcium, titanium and magnesium at lower levels is consistent with the sinking of 
melanite and to some extent of pyroxene crystals. Consideration of the distri- 
bution of alkalis and silica through the suite is more difficult of explanation. The 
uppermost rocks contain the most silica and a remarkable concentration of potash 
such that potash to soda ratios are of the order of 50 : 1. Soda increases at lower 
levels in spite of the fall off in silica. The trend which is defined on a Qz - Ne - Ks 
diagram (Text-fig. 10) could not have been produced by the crystallization and 
removal from the melt of potash-leucite. 



326 PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 

The pseudoleucites in the borolanites consist of potassic feldspar and nephe- 
line-feldspar intergrowths of vermiform type, while in the higher rocks they consist 
of an alkali feldspar and, to a greater or lesser extent, muscovite. Chemically 
the pseudoleucites are more sodic than their host rocks, and they change in com- 
position sympathetically with the rocks. The problem, therefore, is to decide 
what was the original composition of the pseudoleucites and, if different from the 
present composition, by what mechanism was the change achieved. Of the various 
theories put forward to solve the pseudoleucite problem two are paramount. 
Knight (1906) postulated the unmixing on cooling of a soda-rich leucite, while 
Bowen (1928) considered that reaction between leucite crystals and melt is respon- 
sible for pseudoleucite formation. Fudali (1963) has shown that in the system 
quartz - nepheline - kalsilite - water as much as 40 per cent of soda-leucite can 
be held in leucite solid solution, depending on the water vapour pressure, and he 
has demonstrated that pseudoleucites from the Bearpaw Mountains, Montana, are 
derived from the breakdown of soda-leucites. On Text-fig. 11 the maximum ex- 
tent of leucite solid solution determined by Fudali is shown, and one of the analysed 
borolanite pseudoleucites lies close to the extreme soda-leucite composition. 
However, there is no evidence, so far, for extensive, indeed any, solid solution 
between soda-leucite and potash feldspar. Subsolidus breakdown will not, there- 
fore, explain the compositions of the other three Borralan pseudoleucites. Some 
change in their bulk composition must have taken place. 

As already pointed out, the change in composition of the pseudoleucite suite 
cannot be attributed to separation of potash-leucite because the later fractions 
would be enriched in soda, not potash, while if it were postulated either that leucite 
floated to the top of the magma chamber, or that crystallization proceeded from 
the top downwards, then the observed trend in terms of Qz - Ne - Ks would not 
result. Crystallization of soda-leucite, on the other hand, is a reasonable mech- 
anism. As the ultra-potassic pseudoleucite rocks are much less voluminous than 
the borolanites an acceptable primary magma would be a borolanite such as num- 
ber 2 on Text-fig. 11. Separation of leucite containing 30-40 per cent soda-leucite 
molecule from such a magma would enrich the melt in potash and silica, driving 
it towards the orthoclase end of the feldspar join. If the pseudoleucite reaction 
now became operative, reaction of soda-leucite crystals with the melt would pro- 
duce a mixture of alkali feldspar and nepheline, resulting in the addition of further 
sodium to the leucites and enriching the melt still further in potassium. Because 
of the depletion of the melt in sodium the primary crystallizing leucite phase prob- 
ably became increasingly potassic, and this together with the increase of silica 
in the melt would gradually change the pseudoleucite reaction from one producing 
feldspar + nepheline to one producing feldspar only. If there was an increase 
in the water vapour pressure during the differentiation, as seems probable, then 
this would also cause a decrease in the sodium content of the primary leucite, as 
shown by the work of Fudali (1963). This hypothesis therefore requires both the 
production of a soda-leucite, as originally suggested by Knight (1906), and Bowen's 
(1928) pseudoleucite reaction to proceed from the bottom of the magma chamber 
and to work its way through to the top. 



FROM THE BORRALAN COMPLEX, SCOTLAND 327 

Bowen (1928, p. 256) interpreted the layered structure of the Borralan Complex 
as described and illustrated by Shand (1910, Fig. 2) in terms of the gravitational 
settling of leucite, plus the pseudoleucite reaction, resulting in an ultrabasic zone 
at the bottom grading through to a quartz syenite zone at the top. The pseudo- 
leucite suite is in fact intruded by the later syenites (Woolley 1970) and the ultra- 
basic rocks probably do not form a basal layer to the Complex (Parsons 1965, 
p. 57), but Bowen's mechanism, with modifications, is applicable to one group of 
rocks of the intrusion. 

The leucocratic and aegirine-nepheline-analcime borolanites are chemically very 
close to the pseudoleucite borolanites, though free from pseudoleucite. Rocks of 
such compositions would crystallize leucite initially, but the subsequent pseudo- 
leucite reaction and complete recrystallization has destroyed the pseudomorphs. 
It is envisaged that these rocks represent batches of magma which were somehow 
separated from the main mass of borolanite magma, were presumably subjected 
to different physical conditions and were intruded late. The dearth of mafic 
minerals in the leucocratic borolanite suggests their separation by gravitational 
settling, either in situ or prior to intrusion. 

(2) The lower suite 

A number of late mineralogical and textural changes have been described from 
the lower suite. The rocks which are least affected by these changes are the 
patches of pyroxene-biotite syenite, which occasionally carry nepheline, and the 
shonkinite, and these are considered to be remnants of the primary rocks of 
the lower suite. Although Shand (1910, p. 407 ; 1939, p. 416) suggested that 
the vullinite might be a metamorphosed calcareous sediment, or an igneous rock, 
Phemister interpreted it as ' a metamorphosed igneous rock, probably a meso- 
cratic pyroxene-biotite-syenite or porphyrite ' (Macgregor & Phemister 1937, 
p. 45). It is noteworthy that Phemister's suggested precursor to vullinite coin- 
cides exactly with the primary rock of the lower suite indicated by the present work. 

Four principal secondary effects are recognizable among the lower suite rocks : 

(a) ' Borolanitization.' This involves the formation of titanium garnet in rocks 
adjacent to borolanite sheets in the Gorge and boreholes, and the metasomatic 
introduction of garnet along certain layers in the Gorge outcrops. 

(b) The alteration of pyroxene to hornblende. 

(c) The alteration of pyroxene and hornblende to biotite and andradite, with the 
concomitant development of a sodic plagioclase. 

(d) Zeolitization of the feldspar. 

Because of the widespread development of these secondary changes it has proved 
difficult to determine the chemical composition of the primary magma of the lower 
suite. Two analyses, however, seem to be close to it ; the shonkinite (Table VI, 
anal. 18), and the lesser altered of the borehole pyroxene-biotite-andradite syenites 
(Table V, anal. 11) in which nepheline is still recognizable. These two rocks are 
the most undersaturated of the lower suite (Text-fig. 13) and it is apparent from 
Text-figs. 14 and 15 that they are a close match chemically with the group of rocks 



328 PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 

of the Borralan Complex called ledmorites by Shand (1910, p. 384), for which there 
are six chemical analyses available (Shand 1910 ; Tilley 1958 ; Woolley 1965). 
The ledmorites are essentially malignites and shonkinites very similar to the lower 
group pyroxene-biotite syenites. Although there is some variation in the limited 
chemical data on the ledmorites, and although the pyroxene-biotite syenite has 
undoubtedly undergone some chemical change, the mineralogical and chemical 
match is good enough to demonstrate convincingly that the lower suite of rocks of 
the South-eastern Tract are comagmatic with the ledmorites. However, the 
majority of rocks of the lower suite have undergone chemical changes as can be 
seen from Text-fig. 15. Apart from rocks 11 and 18 (Text-fig. 15) all the lower 
suite are only slightly undersaturated, in marked contrast to the highly under- 
saturated ledmorites. Two rocks, the pyroxene microcline xenolith (Table VI, 
anal. 19) and the biotite syenite (Table V, anal. 16), are highly potassic. The 
xenolith was enclosed in pseudoleucite borolanite and would be expected to be 
enriched in potash, while it must be assumed that the biotite syenite was similarly 
metasomatized by a nearby vein of pseudoleucite magma type. The increase 
in silica in the other lower suite rocks is undoubtedly related to the alterations 
involving the development of hornblende and biotite + andradite. 

Microscopic examination of the borehole rocks shows that hornblende replaces 
pyroxene, and even in the purest hornblende types some pyroxene can still be 
found in the cores of crystals. In the vullinite it can be shown that hornblende 
growth is related to the proximity of syenite veins, and these veins comprise part 
of the later suite of syenites and quartz syenites. It is also noteworthy that late, 
spongy hornblende crystals have been found in the pseudoleucite borolanite out- 
crops west of the Gorge, close to the intrusive contact with the later syenites. It 
must be concluded, therefore, that the development of hornblende is a contact 
effect associated with the intrusion of the later syenites. 

The alteration of hornblende and pyroxene to biotite + andradite may be a 
further stage of this contact metamorphism. A similar effect has been found 
among the ledmorites at the west and southwest ends of the Complex (Woolley 
1965), in which the replacement of the pyroxene by biotite + melanite and/or 
andradite garnet can be followed in thin section. This alteration increases towards 
the intrusive contact with the later syenites, and in a zone 100-150 m wide along 
the contact very little fresh pyroxene can be found. The striking parallel between 
this mineral transformation and that found in the lower suite of the South-eastern 
Tract indicates a similar cause, and it is concluded that this is also a contact meta- 
morphic effect of the intrusion of the later syenites, but that it is later than the 
' hornblendization '. 

Returning to the problem of the increase in the degree of silica saturation shown 
by the altered lower suite rocks, the mineralogical evidence indicates the influence 
of the later syenites, and it is, therefore, concluded that the silica increase is part 
of the same contact effect. In Text-fig. 15 the area occupied by the later syenites 
and quartz syenites is shown, and the interpretation of the more silica-rich lower 
suite rocks as reaction between ledmorite type rocks and the later syenites seems 
a reasonable one in terms of this figure. 



FROM THE BORRALAN COMPLEX, SCOTLAND 



329 




Fig. 15. The distribution of the major rock types of the Borralan Complex in the system 
Qz - Ne - Ks. I, pseudoleucite suite ; II, ledmorites ; III, hybrid rocks of the lower 
suite, South-eastern Tract ; IV, later syenites and quartz syenites - based on data of 
Parsons (1972). The squares are the least hybridized pyroxene group rocks (n and 18), 
which plot in the ledmorite field, and the biotite syenite (hornblende group) and pyroxene- 
microcline xenolith which have undergone potash metasomatism. The triangles are 
ledmorite dykes of the Assynt area. Field of leucite (L) etc. as on Text-fig. 10. 



(3) The relationship to the ledmorites 

The four principal groups of rocks which comprise the Borralan intrusion, ex- 
cluding certain ultramafic rocks, are defined in Text-fig. 15 in terms of Qz - Ne - Ks. 
The nepheline syenites of the complex have been shown by Tilley (1958) to include 
two types, a foyaite type and a juvite (juvet) type, and these plot in Text-fig. 15 
in the ledmorite and pseudoleucite suite fields respectively. In a similar plot to 
Text-fig. 15 Tilley (1958, Fig. 1), combining the data then available for the Bor- 
ralan and nearby Loch Ailsh complexes, together with data for the minor intrusions 
of the Assynt region, defined four groups, which correspond in Text-fig. 15 to the 
borolanite part of group I, group II and group IV which Tilley subdivided into 
two separate groups. No data corresponding to group III were then available. 
He concluded that ' Consideration of this analytical plot raises many problems 
relating to the differentiation and evolution of alkali complexes of the Assynt 
type '. A mechanism for the production of group III rocks involving hybridiza- 
tion between II and IV has been outlined above, but the direct genetic relationship 
between the later syenites and the highly undersaturated groups I and II, a prob- 
lem which arises in many syenite complexes, cannot be considered here. However, 
it is believed that the relationship between the ledmorites and the pseudoleucite 
suite can be deduced. 

The extent of primary crystallization of leucite under anhydrous conditions in 
the system Qz - Ne - Ks as determined by Schairer (1957) is indicated in Text- 
fig. 15. The area of this field decreases with increasing water pressure but even 



33Q PSEUDOLEUCITE BOROLANITES AND ASSOCIATED ROCKS 

at iooo bars P H2 o (Fudali 1963) all the pseudoleucite suite, and all but one of 
the ledmorites, lie within the leucite field. The primary ledmorites, that is those 
unaffected by the late replacement of pyroxene by biotite + garnet, contain little 
in the way of hydrous minerals. Modes on eleven of these rocks average less than 
four volume per cent biotite, and some of this is certainly of late development 
related to the contact metamorphism by the later syenites. The ledmorite magma 
was, therefore, essentially a dry one indicating that these rocks would lie well 
within the stability field of leucite. Although pseudoleucites do not occur in the 
ledmorites there is very extensive development of dactylotypic alkali feldspar- 
nepheline intergrowths, which are identical to those found in the borolanite pseudo- 
leucites and in pseudoleucites at other localities, such as the Bearpaw Mountains, 
Montana (Fudali 1963, Plate 2, Figs. 3 & 4). Intergrowths identical to the 
Borralan ones occur in rocks at Kaminak Lake, Northwest Territories, Canada 
(Davidson 1970, Fig. 2), which do not contain pseudoleucites, but Davidson has 
shown convincingly that the feldspar-nepheline intergrowths developed by the 
breakdown of soda-leucite. It seems certain that leucite crystallized initially from 
the ledmorites. The work of Fudali (1963) indicates that the amount of NaAlSi 2 6 
that can be held in leucite-solid solution is increased at low water pressures so that 
it is assumed that soda-leucite separated from a magma of ledmorite type, then 
separation of such a crystal phase, perhaps by upward floating, would produce a 
fraction which, in terms of alkalis and silica, would be compositionally close to 
pseudoleucite borolanite (Text-fig. 15). The melt fraction would move to the 
leucite field boundary and eventually crystallize nepheline and alkali feldspar. 

A plot of MgO against (FeO 4- Fe 2 3 ) (Text-fig. 14) shows that apart from 
three rocks which are enriched in MgO there is a fair correlation of the ledmorites 
with the lower suite. They show, however, a totally different distribution from 
the pseudoleucite suite, which is very low in magnesia but shows a relatively smooth 
trend. The contrasting distributions are explicable in terms of the gravitational 
settling of pyroxene, a major mineral in the ledmorites and lower suite, but absent 
or very minor in the pseudoleucite suite. The sinking of pyroxene and the rising 
of soda-leucite is a mechanism which is adequate to explain the chemical differ- 
ences between these two rock groups. It is believed that the differentiation took 
place before intrusion of the separate groups, although in the South-eastern Tract 
the relative positions of the rocks are consistent with an origin by gravitational 
differentiation in situ. This mechanism is close to the original suggestion of Shand 
(1910, p. 413) of gravitational settling of 'denser molecules ', but Shand considered 
that the whole Borralan intrusion could be explained in terms of in situ differen- 
tiation of a single intruded body of magma. 

There are in the Assynt region a number of dykes which are referred to as led- 
morites by Sabine (1952), and which can be assigned to the Borralan magmatic 
episode. They extend up to 20 miles from the Complex. Four analyses are avail- 
able (Home & Teall 1892 ; Sabine 1952 ; Tilley 1958) and are plotted on Text- 
figs. 14 and 15. In terms of their alkali ratios and degree of silica undersaturation 
they are clearly comparable with the type ledmorites, but they have low MgO 
values similar to those of the pseudoleucite borolanites. The low MgO values of 



FROM THE BORRALAN COMPLEX, SCOTLAND 331 

the dykes suggests some loss of pyroxene, probably by gravitational settling, during 
the lateral movement of this magma fraction away from the Borralan centre. 



VIII. ACKNOWLEDGEMENTS 

I am grateful to Dr R. H. Cummings of the Robertson Research Company for 
supplying material from the boreholes in the Aultivullin Quarry. Professor C. E. 
Tilley kindly gave a fragment from one of Shand's described borolanite specimens. 
Dr Ian Parsons allowed the use, before publication, of his chemical data on the 
later syenites. The chemical analyses were made by Mrs J. Banham, V. K. Din 
and C. J. Elliott, and the electron-probe analyses by R. F. Symes. Dr D. R. C. 
Kempe helped with the diffractometer work on feldspars which were separated 
by Mrs A. M. Rocks, and Miss V. Jones drafted the diagrams. The work was 
begun at Bedford College, University of London, under the supervision of Pro- 
fessor B. C. King whose help is gratefully acknowledged. I wish to express my 
appreciation to my colleague Dr A. C. Bishop who read the manuscript and made 
numerous suggestions for its improvement. 



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geol. Surv. Summ. Prog. 3 : 58-61. 
Prider, R. T. i960. The leucite lamproites of the Fitzroy Basin, Western Australia. /. 

geol. Soc. Aust. 6 : 71 -118. 
Sabine, P. A. 1950. The optical properties and composition of the acmitic pyroxenes. 

Mineralog. Mag. 29 : 113-125. 
1952. The ledmorite dike of Achmelvich, near Lochinver, Sutherland. Mineralog. 

Mag. 29 : 827-832. 
1953- The petrography and geological significance of the Post-Cambrian minor in- 
trusions of Assynt and the adjoining districts of North-west Scotland. Q. Jl geol. Soc. 

Lond. 109 : 137-171. 
Schairer, J. F. 1957. Melting relations of the common rock-forming oxides. /. Am. 

Ceram. Soc. 40 : 215-235. 
Shand, S. J. 1906. Ueber Borolanit und die Gesteine des Cnoc-na-Sroine-Massivs in Nord- 

Schottland. Neues Jb. Miner. Geol. Paldont. Beil Bd 22 : 413-453. 
1909. On borolanite and its associates in Assynt. (Preliminary communication.) 

Trans. Edinb. geol. Soc. 9 : 202-215. 
1910. On borolanite and its associates in Assynt. (Second communication.) Trans. 

Edinb. geol. Soc. 9 : 376-416. 

1939. Loch Borolan laccolith, Northwest Scotland. /. Geol. 47 : 408-420. 

Smith, W. Campbell. 1909. On the composition of ' borolanite ' from Am Meallan, Ross- 
shire. Geol. Mag. (N.S.) dec. 5, 6 : 152-157. 
Steiger, R. H. & Hart, S. R. 1967. The microcline-orthoclase transition within a contact 

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Stewart, F. H. 1941. On sulphatic cancrinite and analcime (eudnophite) from Loch Boro- 
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Stumpfl, E. F. 1 961. Contribution to the study of ore minerals in some igneous rocks from 

Assynt. Mineralog. Mag. 32 : 767-777. 
Teall, J. J. H. 1900. On nepheline-syenite and its associates in the North-west of Scotland. 

Geol. Mag. (N.S.) dec. 4, 7 : 385-392. 
1907. Post-Cambrian igneous rocks of older date than the great thrust-movements of 

the region : their petrography. In Peach, B. N., Horne, J., Teall, J. J. H. et al. 1907. 

The geological structure of the North-west Highlands of Scotland. Mem. geol. Surv. U.K. 

pp. 440-452. 



FROM THE BORRALAN COMPLEX, SCOTLAND 333 

Tilley, C. E. 1956. Nepheline associations. Verh. K. ned. geol. mijnb. Genoot. 16 : 403-413. 

1957- Problems of alkali rock genesis. Q. Jl geol. Soc. Lond. 113 : 323-358. 

1958. Some new chemical data on assemblages of the Assynt alkali suite. Trans. 

Edinb. geol. Soc. 17 : 156-164. 

Tilling, R. I. 1968. Zonal distribution of variations in structural state of alkali feldspar 
within the Rader Creek Pluton, Boulder Batholith, Montana. /. Petrology 9 : 331-357. 

Tyler, R. C. & King, B. C. 1967. The pyroxenes of the alkaline igneous complexes of eastern 
Uganda. Miner alog. Mag. 36 : 5-21. 

Wade, A. & Prider, R. T. 1940. The leucite-bearing rocks of the West Kimberley area. 
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Washington, H. S. 1906. The Roman Comagmatic Region, pp. 1-199. Carnegie Insti- 
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Woolley, A. R. 1965. The Loch Borralan Alkaline Igneous Complex. Unpublished Ph.D. 
Thesis, University of London. 

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7 : 171-182. 

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Borralan Complex, Scotland. Mineralog. Mag. 38: 819-836. 
Wright, T. L. 1967. The microcline-orthoclase transformation in the contact aureole of the 

Eldora Stock, Colorado. Am. Miner. 52 : n 7- 136. 
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Geogr. 24 : 93-100. 



Dr. A. R. Woolley 

Department of Mineralogy 

British Museum (Natural History) 

Cromwell Road 

London SW7 5BD 



PLATE 16 

Fig. i. Pseudoleucites and pseudomorphs after pyroxene (?) (black, towards top) in a 
potassic feldspar-biotite-magnetite rock. Ordinary light; scale line, 3mm [1972, P8 (311)]. 

Fig. 2. A pseudomorph of biotite and magnetite after pyroxene (?), in a potassic feldspar- 
biotite-magnetite rock. Ordinary light ; scale line, 0-5 mm [1972, P8 (311)]. 

Fig. 3. Sheaves of muscovite enclosed by a plate of potassic feldspar, in a potassic feldspar- 
muscovite rock. Crossed nicols ; scale line, 0-5 mm [1972, P8 (131)]. 

Fig. 4. Fine-grained borolanite from close to contact with biotite-magnetite group. Black, 
magnetite ; dark grey, high relief, melanite and sphene ; medium grey, biotite ; white, potassic 
feldspar. Ordinary light ; scale line, o-i mm [1972, P8 (364)]. 



Bull. Br. Mus. nat. Hist. (Miner.) 2, 6 



PLATE 16 

















JWsfH 

•. -t- 



k \ *. J' 







PLATE 17 

Fig. 1. A pseudoleucite in a borolanite has been overgrown by a large potassic feldspar 
crystal, but the original edge of the leucite is sharply defined by ' dust ', which is probably a 
remnant of the pre-recrystallization leucite porphyry. Biotite, dark grey. Crossed nicols; 
scale line, 0-05 mm [1972, P8 (344)]. 

Fig. 2. A large plate of potassic feldspar has overgrown a pseudoleucite (to the left) and 
matrix in a borolanite. Nepheline (grey, high relief) occurs in the pseudoleucite. Biotite 
dark grey ; melanite, dark grey, high relief . Ordinary light ; scale line, o-i mm [1972, P8 (363)] 

Fig. 3. Melanite (high relief) has grown within biotite (grey, low relief) in a pseudoleucite 
borolanite. Potassic feldspar is off white. Ordinary light ; scale line, o-i mm [1972, P8 (285)] 

Fig. 4. Biotite (grey) and melanite (high relief) pseudomorph pyroxene and are poikilitically 
enclosed bv potassic feldspar in a borolanite. Ordinary light ; scale line, o-i mm [1972 
P8 (124)]. " 



Bull. By. Mas. not. Hist. (Miner.) 2, 6 



PLATE 17 






**««■ 




PLATE 18 

Fig. i. Nepheline in dactylotypic intergrowth with potassic feldspar. Pseudoleucite 
borolanite. Crossed nicols ; scale line, o-i mm [1972, P8 (285)]. 

Fig. 2. Nepheline and potassic feldspar in dactylotypic intergrowth. Some feldspar 
grains (slightly strained) are free of nepheline, and there is a distinct tendency for nepheline 
to be concentrated at feldspar grain boundaries. Pseudoleucite borolanite. Crossed nicols ; 
scale line, o-i mm [1972, P8 (307)]. 

Fig. 3. Nepheline (dark grey) and potassic feldspar (grey) showing complex intergrowth. 
There is probably a replacement relationship, but the direction of this replacement is not 
readily apparent. The white flecks in the nepheline are cancrinite. Pseudoleucite borolanite. 
Crossed nicols ; scale line, o-i mm [1972, P8 (363)]. 

Fig. 4. Part of a Carlsbad twinned porphyroblast of potassic feldspar overgrowing a rock 
of feldspar, nepheline, aegirine, melanite, biotite and ore. All these minerals form inclusions 
in the porphyroblastic feldspar. Aegirine-nepheline-analcime borolanite. Crossed nicols ; 
scale line, o-i mm [1972, P8 (334)]. 



Bull. Br. Mus. nat. Hist. (Miner.) 2, 6 



PLATE 18 




PLATE 19 

Fig. i. Pyroxene-microcline xenoliths included in pseudoleucite borolanite. Aultivullin 
Quarry. The large xenolith is approximately 0-5 m in diameter. 

Fig. 2. Pyroxene-microcline xenolith. From an inclusion in pseudoleucite borolanite, 
Aultivullin Quarry. Ordinary light ; scale line, 0-2 mm [1972, P8 (173)]. 

Fig. 3. Shonkinite, Aultivullin Gorge. Pyroxenes (medium grey) are set in a matrix of 
potassic feldspar and zeolite (pale grev to white). Ordinary light ; scale line, 0-2 mm [1972, 
P8 (399)]- 



Bull. Br. Mm. nat. Hist. (Miner.) 2, 6 



PLATE 19 









PLATE 20 

Fig. 1. Vullinite, Allt a'Mhuilinn. Aligned biotites (dark grey) are cut across by a crystal 
of plagioclase (dotted outline in ink) which has inclusions of biotite and pyroxene (high relief). 
Larger pyroxenes are also apparent. Feldspar, oft white. Ordinary light ; scale line, o-i mm 
[1972, P8 (68)]. 

Fig. 2. A cluster of small pyroxene grains (grey) is surrounded by a single large, ragged 
hornblende (dark grev to black) which is replacing them. Hornblende-pvroxene-andradite 
syenite. Borehole 1 at 40 feet (12-19 m), Aultivullin Quarry. Ordinary light; scale line, 
0-2 mm [1969, P17 (51)]. 

Fig. 3. Melanite (dark grev, high relief) poikilitically encloses, and extends between, potassic 
feldspar grains (off white). Biotite, medium grey. Granular borolanite. Borehole iA at 
61 feet (18-59 m), Aultivullin Quarry. Ordinary light ; scale line, 0-2 mm [1969, P17 (9)]. 



Bull. Br. Mus. not. Hist. (Miner.) 2, 6 



PLATE 20 








PLATE 21 

Fig. i. Andradite-biotite syenite showing the typical ' ragged ' texture. Rectangular 
shapes towards the top are pseudomorphs after pyroxene. Andradite, grey, high relief ; 
biotite, grey, low relief ; magnetite, black ; feldspar, white to pale grey. Borehole 2 at 50 feet 
(15-24 m), Aultivullin Quarry. Ordinary light ; scale line, 0-5 mm [1969, P17 (36)]. 

Fig. 2. Hornblende syenite, towards the top, is being altered to andradite-biotite syenite, 
towards the bottom. The relatively simple crystal boundaries of the hornblende (very dark 
grey) syenite contrast markedly with the ragged texture of the andradite-biotite syenite. 
Andradite, dark grey, high relief ; biotite, grey, low relief ; feldspar, white to pale grey. Bore- 
hole 1 at 90 feet (27-43 m ). Aultivullin Quarrv. Ordinarv light ; scale line, 0-2 mm [1969, 
P17 (60)]. 

Fig. 3. Hornblende-pyroxene syenite. Euhedral to sudhedral hornblendes, with occasional 
pyroxene cores (lower centre), and individual pyroxene grains. Hornblende, dark grey ; 
pyroxene, grey, high relief ; feldspar, white to pale grev. Borehole 1 at 145 feet (44-20 m), 
Aultivullin Quarry. Ordinary light ; scale line, 0-2 mm [1969, P17 (71)]. 

Fig. 4. Hornblende-pyroxene syenite showing the complex, poikilitic forms of the horn- 
blendes. Hornblende, dark grey ; pyroxene, grey ; ore, black ; feldspar, white to pale grey. 
Borehole 1 at 45 feet (13-72 m), Aultivullin Quarrv. Ordinarv light ; scale line, 0-2 mm 
[1969, P17 (52)]. 



Bull. Br. Mus. not. Hist. (Miner.) 2, 6 



PLATE 21 




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